Learn Russian: Кто-то ищет команду, которой ты уделяешь время. - Someone is looking for the team you devote time to.

In the second part of the sentence, the verb уделяешь (you devote) requires the Dative case for the receiver of the action (devoting time to the team). Because которой refers back to the feminine noun команда (team), it takes the feminine Dative form.

Why is команда in the Accusative case (команду) if the lesson is about the Dative case?

The word команду belongs to the first half of the sentence, where it is the direct object of ищет (is looking for). A relative clause has its own independent grammar rules. So the team is Accusative in the main sentence, but the 'which' referring to it is Dative in the second part!

Why can't I just skip которой like we skip 'that' or 'which' in English?

In English, we often drop connecting words to say things like 'the team you devote time to'. In Russian, you can never skip the connecting word который. You must always include it.

What is the infinitive of ищет? It looks so different!

The infinitive is искать (to look for). It has a consonant mutation where the ск changes to щ in its present tense forms, giving us forms like я ищу (I look for) and он ищет (he looks for).

Is время in the Nominative case here?