What mattress is best to pair with the NORESUND bed frame from IKEA?
It seems like thin mattresses don’t go very well because the bed seems very low.. I don’t know which mattress to choose. And also, what is a mattress base used for?
It would be preferable if the items recommended are from IKEA.

You really should check with the IKEA personnel to get the correct answer that you are looking for.
If you have a store in your area please visit them in person.
The mattress base is obviously for the mattress to sit on which gives it extra support.