To summarize my knowledge of bedrooms:
When you make a bedroom, and then leave it, the first dwarf to sleep there will claim it as theirs. When they die, this room is made available again. Because of this, you never have to assign bedrooms for commoners, just make a lot of bedrooms and let them sort it out. They seem to prefer sleeping in unassigned bedrooms more than dormitories, so bedrooms get claimed quickly.
Any chests, armor stands, weapon racks, cabinets, and other traditional furniture will become that dwarf's, under the room usage. So, if there's an armor stand in the room, and you make it a bedroom, the armor stand is already included. You can see this when you look at a dwarf, and it'll list their owned items, including "Meager Bedroom, 1 Armor Stand, 1 Chest, etc". These count for noble's needs, as they'll demand chests, cabinets, and armor stands/weapon racks as part of their owned things. It appears that having a private dining room with things in it also makes it owned by the dining room's owner, so having a grand dining room with armor stands as decoration for a noble, with make those stands owned by said noble.
You can assign armor racks to individuals, and they'll presumably store their armor there while off-duty, however this is buggy at best, because soldiers do a poor job of removing and re-arming their equipment. It's pretty much always better to leave them armed at all times and not worry about it. An armor rack inside a bedroom will just be decoration until you make a barracks/armory out of it, in this case an armory since it shouldn't have training active.
As for which bed they sleep in, I'm not sure, although I think children of that dwarf will use the extra beds, so family bedrooms should likely get a few more? Tables and chairs, I usually make a dining room/study out of, so I don't know if they're claimed by default as part of the bedroom.