You can download buildings as polygons from OpenStreetMap (OSM) - worldwide. Use e.g. Quick OSM plugin (screenshot 1).
If you have an attribute for the hight of the building, you can use this to get 3D rendering of the polygon. Some OSM buildings already contain an attribute value for
building:levels. Use this in
3D View of
Layer Styling (or create a new field and and height-values), where you can set this attribute for
Extrusion. Then open
Menu View / New 3D Map View: the buildings that contain a value for the
building:levels attribute are now shown in 3D (see screenshot 2).
Another option would be to combine the polygons with a Digital Surface Model (high resolution, best get LIDAR data) to get the 3D shapes of the buildings from there.
Screenshot 1: downloaded building polygons from OSM with QuickOSM
Screenshot 2: 3D extrusion of buildings, based on OSM attribute