Depends on the journal.

Academics usually do the peer reviewing and sometimes the editing (which is almost certainly a waste of their talents as a good editor is cheaper and more efficient at editing than some Biology PhD guy).

Most of the more prestigious journals (e.g., the ones people are paying for) are supported by publishing house staff doing the sort of stuff I mentioned -- or, more likely, managing the vendors in India who are doing the stuff I mentioned.

Source: I worked in scientific publishing for many years. Happy to be out of it...