Learn Finnish: Minä olen kotona, ja he ovat täällä.

In standard Finnish, you usually must use the third-person pronouns (hän for he/she, he for they). While you can often drop the pronoun for I, you, we, and you (plural) because the verb ending makes the subject obvious, you generally cannot drop it for the third person.

Can I use he to talk about animals or objects, like saying 'they are here' about my keys?

No, he is strictly used for people. If you want to say 'they are' when talking about animals or things, you must use ne ovat, where ne is the plural pronoun for non-humans.

Why is the word for home kotona instead of just koti?

Kotona is a special location form that means 'at home'. The base word is koti (home), but Finnish uses the ending -na here to show a state of being in a place. If you said minä olen koti, it would sound like you are literally a building!

Why is there a comma before ja (and)?

In Finnish grammar, a comma is required before ja when it connects two independent clauses. Since Minä olen kotona and he ovat täällä are both complete sentences with their own subjects and verbs, they must be separated by a comma.