Learn Hardcore Finnish: Kirjoitan näppäimistöllä raportin ja tallennan jokaisen tiedoston työpöydälle. - I type the report with the keyboard and save every file to the desktop.

The base word is näppäimistö (keyboard).
The ending -llä / -llä is the adessive case, which among other things often marks the instrument used to do something.

So näppäimistöllä literally means on the keyboard, but functionally it means with a keyboard / using a keyboard.
This is the same pattern as:

kynä → kynällä = with a pen
puhelin → puhelimella = with a phone (e.g. talk on the phone)

So Kirjoitan näppäimistöllä = I write using a keyboard.

Why is it raportin and not raportti?

Raportin is the object in the “total object” form (which in singular looks like the genitive: -n).

When the object is complete / fully affected by the action, Finnish typically uses this form:

Kirjoitan raportin. = I (will) write the whole report.
Luen kirjan. = I (will) read the whole book.

If you said Kirjoitan raportti, that would be ungrammatical. The object must be in an object case, not bare nominative in this sentence.

What would be the difference if I said Kirjoitan raporttia instead of Kirjoitan raportin?

Raporttia is the partitive form and usually implies an ongoing or incomplete action:

Kirjoitan raporttia. = I am writing a report (I’m in the middle of it; not necessarily finishing it).
Kirjoitan raportin. = I will write / I’m writing the whole report (there is a sense of completion or goal).

So raportin presents the action as a bounded, completed task, while raporttia focuses on the process without guaranteeing completion.

Why do both words in jokaisen tiedoston have the -n ending?

Jokaisen tiedoston is a genitive + genitive structure:

jokainen (each, every) → jokaisen (genitive)
tiedosto (file) → tiedoston (genitive / total object)

In this sentence, jokaisen tiedoston functions as a single object phrase: each file / every file.
When a quantifier like jokainen is declined, the noun it modifies usually agrees with it in case:

jokainen tiedosto (nominative)
jokaisen tiedoston (genitive / total object)
jokaista tiedostoa (partitive)

Here the verb tallennan presents the action as completed for each file, so the object phrase appears in the total object form, which looks genitive in singular.

Could I say tallennan jokaisen tiedoston työpöydällä instead of työpöydälle?

You should not in this context; it would change or break the meaning.

työpöydälle = onto the desktop (allative case, movement toward a surface)
työpöydällä = on the desktop (adessive case, being on a surface)

Tallennan jokaisen tiedoston työpöydälle means I save each file *onto the desktop (movement of the file to that location).
*Työpöydällä would describe location, not direction, and would sound wrong here: you are not saving on the desktop in the static sense; you are saving to it.

Why is it työpöydälle and not something like työpöytään?

Työpöytä (desktop / work desk) forms:

työpöydällä = on the desktop (adessive)
työpöydälle = onto the desktop (allative)
työpöytään = into the desktop (illative)

For flat surfaces where we conceptualize things as being on top of them, Finnish typically uses the -lla / -lle series:

pöydällä = on the table
pöydälle = onto the table
työpöydällä = on the desktop
työpöydälle = onto the desktop

Työpöytään would suggest movement into something (like into a box), which doesn’t fit the metaphor of the desktop as a surface.

Does työpöytä always mean the computer desktop?

No. Literally, työpöytä is:

työ = work
pöytä = table

So it literally means work table / work desk.
In computer contexts, it also means desktop (the screen area where your icons and files appear). Context tells you which one is meant.

In this sentence with tiedoston and tallennan, it clearly refers to the computer desktop.

Why is the verb in present tense (Kirjoitan, tallennan) when English might say “I will write and save”?

Finnish present tense is often used where English uses both present and future. Context and adverbs give the time reference.

Kirjoitan raportin huomenna. = I will write the report tomorrow.
Tallennan jokaisen tiedoston. = I will save each file. / I save each file.

There is no separate future tense in Finnish. So your sentence in Finnish present tense naturally covers a future or habitual meaning, depending on context.

Is the word order fixed? Could I say Kirjoitan raportin näppäimistöllä or Tallennan työpöydälle jokaisen tiedoston?

Finnish word order is relatively flexible, and both alternatives are grammatical, but the default neutral orders are:

Kirjoitan näppäimistöllä raportin.
Tallennan jokaisen tiedoston työpöydälle.

If you say Kirjoitan raportin näppäimistöllä, you slightly emphasize the object (the report).
If you say Tallennan työpöydälle jokaisen tiedoston, you emphasize the destination (the desktop) more.

So you can change the order, but it subtly changes focus / emphasis.

Why do we use ja only once, before tallennan, and not repeat the subject?

In Finnish, when two verbs share the same subject, you usually:

state the subject once
conjugate both verbs
connect them with ja (and)

So:

Kirjoitan näppäimistöllä raportin ja tallennan jokaisen tiedoston työpöydälle.
= I write the report … and (I) save each file …

Repeating the subject minä before both verbs is possible but unnecessary and would sound more emphatic:

Minä kirjoitan… ja minä tallennan…

Omitting the second minä is the normal style.

Why are there no words like a or the (articles) in this Finnish sentence?

Finnish has no articles like English a / an / the.
The specificity of nouns (a report vs the report) is understood from:

context
word order
cases (e.g. total vs partitive object)
adjectives or other modifiers

So raportin here can be understood as “the report” or “a report” depending on the wider context, but Finnish grammar itself does not mark that contrast with a separate word.

What exactly does jokaisen add compared to just saying tallennan tiedoston työpöydälle?

Tallennan tiedoston työpöydälle. = I save the file (a specific file) onto the desktop.
Tallennan jokaisen tiedoston työpöydälle. = I save each/every file onto the desktop.

Jokaisen emphasizes quantity and exhaustiveness: you are not just saving one file, but all of them, one by one. It adds the meaning of every single one.

Is there any difference in meaning between jokaisen tiedoston and kaikki tiedostot here?

Both can cover a similar idea, but the nuance is slightly different:

kaikki tiedostot = all the files (as a group, totality)
jokaisen tiedoston = each/every file (emphasizes individual items)

So:

Tallennan kaikki tiedostot työpöydälle. = I save all the files onto the desktop.
Tallennan jokaisen tiedoston työpöydälle. = I save each file onto the desktop (slightly stronger feeling of doing it for every single file individually).