Learn Hardcore Russian: В этой биографии подробно описано его детство, учёба и профессия. - In this biography his childhood, studies, and profession are described in detail.

So биографии here is the prepositional case, governed by the preposition в, meaning “in this biography” (i.e., in this written life story).

Using на биографии would be wrong in this context; на is used with surfaces, events, some institutions, etc., but not with texts in this meaning.

What exactly is описано grammatically? Is it a verb? A participle?

Описано is a short passive participle (краткое страдательное причастие) from the perfective verb описать (“to describe (completely)”).

Full breakdown:

Infinitive: описать – to describe (perfective)
Past passive participle (full form): описанное (neuter), описанный, описанная, описанные
Short passive participle: описано (neuter), описан, описана, описаны

In this sentence it functions like a predicative, similar to “is described” / “has been described”:

подробно описано ≈ “is described in detail / is described thoroughly”

So it’s not a normal conjugated verb form like описал / описали, but a short passive participle used in a predicate position.

Why is it описано (neuter singular) and not plural описаны, since there are three things: детство, учёба и профессия?

This sentence is built as an impersonal passive construction, where there is no grammatical subject. In such sentences, Russian often uses neuter singular as a “default” predicate form:

В этой биографии подробно описано …
Literally: “In this biography, [it] is described in detail, his childhood, studies, and profession.”

The phrase его детство, учёба и профессия is what is being described, but grammatically it looks more like a detached “what” rather than a clear, agreeing subject.

You can make them the grammatical subject and use the plural:

Его детство, учёба и профессия подробно описаны в этой биографии.

Here, детство, учёба и профессия are clearly the subject (three items), so the predicate becomes описаны (plural).

Both versions are correct; the original impersonal описано sounds very natural when you present the fact of detailed description rather than emphasizing the three items as subjects.

Can I rephrase the sentence as Его детство, учёба и профессия подробно описаны в этой биографии? Does it change the meaning?

Yes, that rephrasing is grammatically correct and very natural:

Его детство, учёба и профессия подробно описаны в этой биографии.

Differences in nuance:

Original:
В этой биографии подробно описано его детство, учёба и профессия.
– Slightly more emphasis on the biography itself and the fact that it contains a detailed description.
Rephrased:
Его детство, учёба и профессия подробно описаны в этой биографии.
– Slightly more emphasis on his childhood, studies and profession as the things that are described.

Meaning-wise they are almost the same; it’s mostly a matter of focus and style.

What does подробно mean exactly, and how is it different from подробнее?

подробно is an adverb meaning “in detail, thoroughly”:
- подробно описано – “described in detail / described thoroughly”
подробнее is the comparative form: “in more detail, more thoroughly”.

Examples:

Расскажите подробнее. – “Tell (me) in more detail.”
В книге подробно описано его детство. – “His childhood is described in detail in the book.”

So in the given sentence, подробно just states that the description is detailed, without comparison.

What is учёба exactly? Does it just mean “study”, or something more specific?

Учёба is a noun derived from учиться (“to study”). It usually refers to:

the process of studying,
the period of one’s studies, or
schooling / education in the practical, everyday sense.

English equivalents depend on context: studies, schooling, time at school/university, education.

Examples:

Моя учёба заняла пять лет. – “My studies took five years.”
Он уехал в Москву на учёбу. – “He went to Moscow to study / for his studies.”
У неё была тяжёлая учёба. – “Her studies were difficult.”

In the sentence его детство, учёба и профессия, учёба means roughly “his years of studying / his education period”.

What case are детство, учёба и профессия in here? Are they objects?

Forms:

детство – neuter singular, nominative form
учёба – feminine singular, nominative form
профессия – feminine singular, nominative form

In a fully personal version like:

Его детство, учёба и профессия подробно описаны.

they would clearly be nominative plural as the subject (“childhood, studies, and profession are described”).

In the original impersonal construction:

В этой биографии подробно описано его детство, учёба и профессия.

they are still in nominative form, but behave more like what is described, without acting as a straightforward grammatical subject. Russian allows this kind of structure, where the “logical object” appears in nominative with an impersonal predicate.

How do I say “my childhood, studies and career” instead of “his”? How does его work here?

Его is a third-person possessive pronoun: “his” or “its”. It is indeclinable: it looks the same in all cases.

To change the possessor:

его детство, учёба и профессия – his childhood, studies and profession
моё детство, моя учёба и моя профессия – my childhood, my studies and my profession
её детство, её учёба и её профессия – her childhood, her studies and her profession
их детство, их учёба и их профессия – their childhood, studies and profession

Note gender/number agreement with мой / моя / моё etc., but его itself does not change form, regardless of gender or case:

его детство, его учёба, его профессия – always его.

Could I say своё детство instead of его детство in this sentence?

You could say своё детство in a sentence where the subject is the same person whose childhood is being described:

В своей биографии он подробно описал своё детство, учёбу и профессию.
– “In his (own) autobiography he described his childhood, studies and profession in detail.”

Here, он (he) is the subject, and своё refers back to that subject (reflexive possessive).

In the original sentence:

В этой биографии подробно описано его детство, учёба и профессия.

there is no explicit subject; it’s an impersonal passive construction. Using его is natural because it just says “his/that person’s childhood…”, without tying it to a grammatical subject.

Using своё in the original structure would sound odd and unclear, because there is no clear subject for своё to refer back to.

How is the stress and pronunciation in this sentence? Any tricky words?

Approximate stress (stressed syllables in caps):

В Этой биогРАфии подрОбно опиСАно егО ДЕтство, учЁба и профЕссия.

Word by word:

э́той – É-toy (stress on э)
биогра́фии – bee-uh-GRA-fee-ee (stress on гРа)
подро́бно – pa-DROB-na (stress on дроб)
описа́но – a-pee-SA-na (stress on са)
его́ – ye-VO (stress on во, and the initial е is pronounced [йи]: йивО)
де́тство – DYET-stva (stress on дет)
учёба – u-CHYO-ba (stress on чё, ё is always stressed)
профе́ссия – pra-FE-ssiya (stress on фе; double с is just [s], not long in normal speech)

Most common learner issues here:

Remember его is pronounced йиво́, not е-го́.
Remember учёба has ё, always stressed.
Don’t move the stress in биогра́фии or профе́ссия.

Why don’t we see a separate word for “is” in Russian, like “is described”? Where is the “is”?

In Russian, in the present tense, the verb “to be” (быть) is normally omitted in simple statements:

Он врач. – literally “He doctor.” (means “He is a doctor.”)
Книга интересная. – “The book (is) interesting.”

In passive constructions like this, the short passive participle itself (here описано) functions as the predicate, so no separate “is” is needed:

подробно описано – literally “(in detail) described”
≈ “is described in detail / has been described in detail.”

So English needs “is” or “has been”, but Russian expresses the state just with the participle and adverb.

Is there any difference between using описано and описывается here?

Yes, there is a nuance:

описано (short passive participle of perfective описать)
– result-focused, implies the description is completed or fully present:
В этой биографии подробно описано…
≈ “In this biography, his childhood, studies, and profession are (fully) described in detail.”
описывается (3rd person singular, present tense, imperfective описывать)
– process or general fact, like “is (being) described / is described (as a regular fact)”:
- В этой биографии подробно описывается его детство.
  – “His childhood is described in detail in this biography.” (focus more on describing as an ongoing action/characteristic of the text)

In your sentence, описано is more natural because we’re talking about the completed, existing description inside the biography.