Do generalist journals try to make sure every subfield is adequately represented? If so, how?
I think you are suggesting that there might be some sort of quota system and I doubt that such a thing is done, or even viable. You are interested in fairly broad areas of specialization within mathematics and, while I'm not in the game anymore, I have to guess that there are plenty of papers submitted in each of the categories (or they would be dropped or coalesced).
But the editors, and especially the editor in chief, is most interested in publishing the highest quality papers. Those that are interesting. Those that make serious advances. Those that suggest new ways of approaching mathematical questions. The &quot;balance&quot; will come naturally, due to the sheer number of papers.
But, if I'm the editor in chief (as I'm not), and I have two papers, one in a recently popular field and one in a less popular one, I'm going to choose the better paper. The other consideration would be far down my list of criteria. If I have a great paper and an ok but not earth shattering one, it is an easy choice.
And if there is some imbalance between the number of papers published in a subfield, I'll have to guess that it is related to the number of practitioners and the current state of the art in that field. Some fields have fewer researchers, and some threads of research get &quot;stuck&quot; for a time. Publishing &quot;extra&quot; papers in that field seems counterproductive at the moment decisions need to be made.

For the specific technical questions about quotas and such, you should ask the EiC of TransAMS, for example.