I recommend Blender over Sketchfab. Sketchfab doesn’t really have enough control for reasonable building interiors.
Then just build walls and doors and trim and all the rest in Blender, in whatever workflow you like best. You could start with a block and subtract out the interiors. You could build walls using boxes. You could draw outlines and extrude them. Cut holes for windows. Weld vertices to make bigger composed objects.
Then, export to Unreal using one of the many Blender to Unreal export tutorials.
Note that collision (if you want the player to be inside the building) will matter. You will either want to build simplified collision proxies out of boxes for each section of wall and ground (so it’s easy to not have collision for doors) or you can turn on “use complex collision as simple” – BUT! complex-as-simple will have a runtime cost if you have too complex a building or too many of them.
Now that I think about it, if your quality goals are low, you could also sculpt using BSP geometry in Unreal itself.
You can add boxes for walls and roofs, and cut holes for windows and doors. Collision will automatically work.
BSP doesn’t give you the best artistic control (hard to add trim sheets and fluting and such) but it’s a great way to get started with a layout, and to learn environment shaping if you’re still learning modeling.
There are plenty of tutorials about brush editing / CSG geometry in Unreal Editor, although most of them are older because it’s an older technology.