Learn Finnish: Tänään kilpailijat luistelevat kilpailussa. - Today the competitors skate in the competition.

They are both built from the verb kilpailla (to compete). Adding the suffix -ja gives you the person doing the action (kilpailija = competitor), while adding -u creates the noun for the event itself (kilpailu = competition).

Why is the verb luistelevat formed with an -e- in the middle?

The dictionary form is luistella (to skate), which is a Type 3 verb. To conjugate Type 3 verbs that end in -lla or -llä, you drop the final -la or -lä, add an -e- to create the stem (luistele-), and then attach your personal ending (here, -vat for "they").

Since this lesson is about superlative adverbs, how would I change this sentence to say they skate the "fastest"?

You would use nopeimmin (fastest), which is the superlative adverb form of nopeasti (fast). You can place it right after the verb: Tänään kilpailijat luistelevat nopeimmin kilpailussa.

Why do we use the inessive case (-ssa) for kilpailussa (in the competition)?