How to level two floors?
Yep, as long as you don't have a door threshold that would be a very likely trip hazard, your plan is almost right. Of course, it would be best to remove any surface reason possible in the high room. However, that floor is tongue &amp; groove 2x4's and may be a ceiling for a room below (as seen in the sunlight areas of your picture)...so, don't screw with it.

Conversely yes, in the low room you'd build a stud frame to mimic joists and then plywood it to match the other room's elevation. Although, there's no need to nail it down nor glue it as the high room's floor edge and the walls would keep it tightly in place...your frame would be the same as a stud wall with a top and bottom plate securing the studs/joists.