Learn Hardcore Finnish: Iltapäivällä katu on täynnä autoja. - In the afternoon the street is full of cars.

Iltapäivällä is iltapäivä (afternoon) + the adessive case ending -lla/-llä. The adessive often means on/at, but it’s also commonly used for time expressions meaning in/during a time period.
So iltapäivällä = in the afternoon / during the afternoon.

Is iltapäivällä “in the afternoon” (one specific afternoon) or “in the afternoons” (habitually)?

By default, iltapäivällä is typically understood as at some point in the afternoon (often one specific situation, depending on context).
If you clearly want the habitual meaning in the afternoons, Finnish often uses iltapäivisin (or iltapäivällä can still work with context, but -isin is clearer for “generally”).

Why is the word order Iltapäivällä katu on…? Could it be different?

Finnish word order is flexible. Putting Iltapäivällä first sets the time frame/topic: “As for the afternoon…”.
You could also say:

Katu on iltapäivällä täynnä autoja. (more neutral “The street is full of cars in the afternoon.”)
Iltapäivällä on katu täynnä autoja. (possible, but more marked/emphatic)

The meaning stays largely the same; the change is mainly about focus and flow.

Why is it katu (nominative) and not kadulla?

Because here the street is the subject: katu on täynnä… = “the street is full of…”. The sentence describes a state of the street itself.

If you use kadulla (adessive), you’re more likely making an existential type sentence:

Kadulla on autoja. = “There are cars on the street.”

So:

katu on täynnä autoja = the street is (very) crowded/full
kadulla on autoja = cars exist/are present on the street (not necessarily “full”)

What is on here? Is it always needed?

on is the 3rd person singular present tense of olla (to be): (it) is.
In Finnish, you normally need the verb olla in this kind of “X is Y” sentence:

katu on täynnä = “the street is full”

You can omit olla only in some special styles (headlines, notes), but in normal speech/writing it stays.

What part of speech is täynnä? Is it an adjective?

täynnä functions like a predicative adjective meaning full (of). It commonly appears with olla:

paikka on täynnä ihmisiä = “the place is full of people”

It behaves a bit like an adjective with a built-in “of”-idea: full of X.

Why is it autoja and not autot or autojen?

autoja is the partitive plural of auto. Finnish often uses the partitive for an indefinite quantity (some/any/a lot of), especially in “full of” type expressions:

täynnä vettä = full of water (partitive)
täynnä ihmisiä = full of people (partitive plural)
täynnä autoja = full of cars (partitive plural)

autot (nominative plural) would sound like a specific, fully identified set (“the cars”), and it doesn’t fit this construction naturally.

Does täynnä autoja imply “many cars” or literally “completely full”?

It strongly suggests a lot of cars, often with the nuance that the street feels packed/crowded. It doesn’t have to mean mathematically “no space left,” but it’s stronger than just “there are many cars.”
A milder alternative is:

Iltapäivällä kadulla on paljon autoja. = “In the afternoon there are many cars on the street.”

How would I negate this sentence?

Use the negative verb ei + the connegative form ole:

Iltapäivällä katu ei ole täynnä autoja. = “In the afternoon the street isn’t full of cars.”

How do I pronounce iltapäivällä (roughly), and where is the stress?

Finnish stress is almost always on the first syllable of the word: IL-ta-päi-väl-lä.
Also note:

ä is like the vowel in cat for many speakers (but more fronted)
ll is a long consonant; hold it a bit longer than a single l
So it’s roughly: IL-ta-pai-väl-lä, with a clearly longer -ll-.