Learn Dutch: Wat zoeken jullie precies? - What are you looking for exactly?

The words welk and welke mean "which", and they are used when you are asking someone to pick from a specific group of choices (like "which book?"). Wat simply means "what". It is used for open-ended questions like this one, where the options are unknown or unlimited.

Why is there no word for "for" in the Dutch sentence?

The Dutch verb zoeken means "to look for" or "to seek". The "for" is already built right into the meaning of the word, so you don't need to add an extra preposition like you do in English.

Why doesn't the sentence say something like "Wat zijn jullie zoeken" to mean "What are you looking"?

English uses "are looking" to show that an action is happening right now, but Dutch usually just uses the simple present tense. Zoeken jullie translates perfectly to both "do you look" and "are you looking".

Could I say "Wat zoek je precies?" instead?

Yes, if you are talking to just one person. Jullie is the plural "you" (like "you guys" or "y'all"), so this sentence is asking multiple people what they are looking for. You would use zoek je to ask one person.