Each country gets one or two commissioners. They are appointed by their respective national governments, subject to approval by the EU Parliament (but rejection is as rare as rocking-horse shit).

Basically, commissioners are usually former senior politicians who can no longer get elected, because they've been disgraced in some way or other. It's a sinecure.

It's almost impossible to dismiss them; the EU Parliament only has the power to dismiss the entire commission. That sounds like a nuclear option that could never be used; but is has been done, exactly once. It was over corruption; the parliament decided that essentially every commissioner was crooked.