The demo is from the old build, while 0.801a and 0.801b is the new remade build for the Unreal 5 engine system. They trying to remake the game with the new system, so there is a lot of new changes coming to it in the future, but right now its just a sort of early test build of the new system that you can try out.
It does not have much but thats why its a early test build.