Learn Finnish: Olen todella pätevä tähän työhön. - I am really qualified for this job.

This is because of the adjective pätevä (qualified/competent). In English, we say qualified for, but Finnish adjective rections don't always match English prepositions. The adjective pätevä strictly requires the target of the qualification to be in the illative case (the "into" case). You just have to memorize the pair: pätevä + illative.

How exactly is the illative ending formed for the word työ?

The noun työ is a short, one-syllable word ending in two vowels (a diphthong). For these words, the illative rule is to add -h, then double the last vowel, and add -n. So, työ + h + ö + n becomes työhön.

Why does the word for "this" (tämä) change to tähän here?

In Finnish, modifier words like "this" must match the grammatical case of the noun they describe. Since työ is in the illative case (työhön), tämä must also be in the illative. The pronoun tämä is a bit irregular; its illative form is tähän. Together they make tähän työhön (into this job).

Could I use tosi instead of todella in this sentence?