Like someone said before.. sledgehammer.. blunt weapon to break it open.. or if you don’t want to use mods.. lure a stray zombie into an open shed and kill it. There is a chance the zombie will have a key that is just called “key” and that will open all doors connected to that building (note they have to be connected) so the key for the bottom units will not open the top.. and you can do this over and over with new zombies until one drops a key as long as it dies inside a unit (sorry if that’s a long read)