Learn Finnish: Mekaanikot ovat varikolla, ja he ovat tosissaan. - The mechanics are in the pit, and they are serious.

The base word is tosi (true). Here it is in the plural inessive case (-i-ssa-) combined with a third-person possessive suffix (-an). Literally it means "in their truths," but Finns use this exact structure as a state adverbial to mean being "serious" or "in earnest" about something.

Would tosissaan change if the subject of the sentence was "I" instead of "they"?

Yes! Because this state adverbial uses a possessive suffix, it must match the subject. If the subject was minä (I), you would use the first-person singular suffix -ni to make tosissani (I am serious). For he (they), the third-person suffix -an is used to make tosissaan.

Why do we use varikolla instead of varikossa if the English translation says "in the pit"?

Many specific places in Finnish conventionally use the outer local cases (-lla for "on" or "at") instead of the inner ones (-ssa for "in"). Varikko (depot, garage, or motorsport pit) is one of those words where you are grammatically "on" it rather than "in" it.

In the second half of the sentence, could we drop the pronoun he and just say "ja ovat tosissaan"?