Learn Hardcore Finnish: Palvelutiski on ulko-oven vieressä, joten löydän sen helposti. - The service counter is next to the front door, so I find it easily.

Palvelutiski is a compound noun: palvelu (service) + tiski (counter/desk). Finnish very often combines nouns into one word rather than using of-phrases like English (service counter). The whole compound behaves like a single noun and takes endings as one unit when needed.

Why is it Palvelutiski on... and not Palvelutiskiä on...?

Because this is a basic “X is Y/at Z” statement. The subject palvelutiski is in the nominative (dictionary form). The verb olla (on) links the subject to a location/expression.
You’d use palvelutiskiä (partitive) in different kinds of sentences, e.g. with “some/any” meaning or existence/quantity nuances, but here it’s a specific counter being located.

What exactly is on here?

on is the 3rd person singular present tense of olla (to be): (it) is.
Finnish doesn’t use an extra word like English there is in this kind of simple location statement; X on + location is enough.

Why is it ulko-oven (with -n)?

ulko-oven is the genitive singular of ulko-ovi (outer door / exterior door). The genitive -n is used because vieressä behaves like a postposition-style location word meaning next to / beside, and it typically takes its “reference noun” in the genitive:

ulko-oven vieressä = next to the outer door (literally: at the side of the outer door)

What is vieressä—is it a postposition or a case form?

It’s originally a noun-like word meaning side, used as a location expression. In this sentence it’s in the adessive case (-ssä/-llä), so vieressä literally means at/on the side.
Structure: [genitive noun] + vieressä is one of the common ways Finnish expresses next to.

Why is it ulko-oven vieressä and not something like vieressä ulko-ovea?

The standard pattern is X:n vieressä (genitive + vieressä). ulko-ovea would be partitive, which doesn’t fit this “beside” construction in normal usage.
Word order can vary for emphasis, but the case pattern stays: ulko-oven vieressä is the neutral, expected form.

What does joten do, and why is there a comma before it?

joten means so / therefore, linking a reason to a result:

Palvelutiski on ulko-oven vieressä, joten löydän sen helposti.
The counter is next to the outer door, so I find it easily.

A comma is normally used before joten because it introduces a consequence clause, similar to how English often uses a comma before so in this meaning.

How do we know who the subject is in löydän if minä isn’t written?

The verb ending shows it. löydän is 1st person singular present: I find. Finnish often omits subject pronouns because the verb form already contains the person/number information:

(minä) löydän = I find

Why is the verb löytää becoming löydän (where does the d come from)?

That’s a common Finnish sound/structure change called consonant gradation. In many verbs, a “strong” consonant changes to a “weaker” one in certain forms. Here:

infinitive: löytää
1st person present: löydän

So t → d in this verb’s present-tense stem in the 1st person singular (and some other forms).

Why is it löydän sen (accusative/genitive-looking form) and not löydän sitä (partitive)?

sen is used when the object is seen as complete/definite: you will find it (the specific counter) as a whole result.
sitä (partitive) would suggest something like an ongoing/indefinite process or “some of it,” which doesn’t fit the idea of successfully locating the counter:

löydän sen = I find it (successfully)
etsin sitä = I’m looking for it (often partitive with ongoing action)

What kind of word is helposti, and where can it go in the sentence?

helposti is an adverb meaning easily. It commonly comes near the verb it modifies:

löydän sen helposti (very natural) You can move it for emphasis, but the neutral placement is as in the sentence.