Learn Finnish: Hän on mitä ammattitaitoisin ihminen ja valmis auttamaan ryhmää. - She is an exceedingly professional person and ready to help the group.

This is the absolute superlative construction your current lesson covers. Here, mitä does not mean "what". Instead, it acts as an intensifier for the superlative adjective ammattitaitoisin. Combined with the nominative noun ihminen, it means she is an "exceedingly professional person" rather than strictly comparing her to a specific group of people.

How is the superlative ammattitaitoisin formed?

The base adjective is ammattitaitoinen (professional or skilled). For words ending in -nen, the stem changes to -se-. When you add the superlative marker -i- and the nominative ending -n, that -se- turns into -si-. Therefore, the stem ammattitaitoise- becomes ammattitaitoisin.

Why is the verb auttamaan in the illative form (ending in -maan)?

This is determined by the adjective valmis (ready). In Finnish, expressing that someone is "ready to do" something requires the third infinitive in the illative case (-maan or -mään). So "ready to help" is always valmis auttamaan.

Why is ryhmää in the partitive case?