Learn Finnish: Pidän moottoriurheilusta, joten olen innoissani.

This is a subject-bound state adverbial. It comes from the noun into (enthusiasm or excitement). It is formed using the plural inessive case (innoissa-, literally "in the enthusiasms") plus the first-person possessive suffix -ni ("my"). You are literally saying "I am in my enthusiasms."

Since innoissani has a possessive suffix, does it change if the subject changes?

Yes! The possessive suffix must always match the subject of the sentence. While "I am excited" is olen innoissani, "you are excited" is olet innoissasi, and "she is excited" is hän on innoissaan.

Why is the word for motorsport (moottoriurheilu) written as moottoriurheilusta?

This is a quick reminder of how the verb pitää works. When pitää means "to like", the thing being liked must always be in the elative case (-sta/-stä).

I've learned that niin can mean "so". Why use joten here?

Joten specifically means "so" in the sense of "therefore" or "as a result". It links a direct cause and effect. Niin often means "so" as an adverb of degree (like "so much" or "so big"), and while sometimes used loosely as a conjunction in spoken language, joten is the standard word for "therefore".