The larger 11 by 26 inch repair had plywood and the plywood subfloor in place, so notso hard to begin this repair.
This smaller repair began as a hole in the floor.
There was a 5 1/2 inch wide beam about 5 inches directly under this void, so I glued 4 pieces of plywood as supports on top of that beam, then slipped these three sections of plywood under the floor to support the subfloor via the joist. I had to cut this plywood base into 3 pieces so that I could drop them into the hole and slide them sideways........ under the underlayment. It wouldn't go in in one piece or even two pieces. (those pieces were totally goobered up with urethane adhesive before inserting them)
Once that was done, I could build up the floor to the correct elevation, then start the repair.