Yes, the roof was leaking, but that was addressed in the 90s and
hasn't been a problem since. The second floor of the building hasn't
been refinished since, so the old wood is still sitting there. The
joists underneath do not appear to be rotten and surprisingly look
pretty good as far as I can see through the holes.

About what happens at the exterior walls, it's solid bonded brick
construction, and there's no exterior wood frame of any kind. Although
I haven't looked closely (and can't at the moment because the old owner
hasn't moved out quite yet), I would assume the joists go straight into
the brick and the subfloor tucks under the plaster interior and stops
when it hits wall. The joists are additionally supported by two very
large steal I-beams that were added later in the life of the building
and also run straight into the brick.

It's designated historic and has been standing for nearly a century
now. It's had an interesting history, and I'm hoping to make it nice
and live-able again.