Just to elaborate a bit, UEd always used a solid map initially that you’d carve spaces into by subtracting cubes and shapes out of it. I believe this is optional now or so I seem to remember. You can either do this by subtracting rooms and corridors then decorating with static meshes or you can initially subtract a large area the size of a zone or the map, then add polygons, smeshes, objects, terrain and a skybox to your liking.
To try out a very basic map, just subtract a box of about 2000^3 and place a few ambient lights on the ground, a few player starts, some path nodes for the AI to navigate along and a weapon locker wouldn’t hurt if it’s not too fiddly to set up (I forget if it needs any setting up first). Then you can ‘build all’ from the drop down menu and try the cube out. When it’s loaded bring down the console with ~ and type addbots 10. One other thing I can’t remember how to do is tweak this so you start this with the desired weapons, bots, gravity etc, so you’ll be trying the map with the assault rifle and shield gun. It’s most likely alterations to a config file somewhere, worth looking up how to set up starting conditions if desired.