Step 1.
Use same steps as to find city limit.
https://transparentgov.net/cleargov1/1818/how-to-find-any-city-boundary
Step 2.
Or if you already have city limit, you can just search 'building'
Step 3.
San bernadino county building foot print is here
https://transparentgov.net/json2tree/esri/server/folder2.html?url=https%3A%2F%2Fservices.arcgis.com%2FaA3snZwJfFkVyDuP%2FArcGIS%2Frest%2Fservices&org=https%3A%2F%2Fservices.arcgis.com%2FaA3snZwJfFkVyDuP%2FArcGIS%2Frest%2Fservices&arcgis_online_token=&select_folder=9&select_layer=0&select_folder_text=2D_Building_Footprints_2021+%3Csup%3EFeatureServer%3C%2Fsup%3E&select_layer_text=0+%26%23x21E2%3B+SanBernardino_2021_Buildings+%3Csup%3EFeature+Layer%3Csub%3E+esriGeometryPolygon%3C%2Fsub%3E%3C%2Fsup%3E
Step 4.
orange county building foot print is here
https://transparentgov.net/json2tree/esri/server/folder2.html?org=https%3A%2F%2Focgis.com%2Farcpub%2Frest%2Fservices&url=https%3A%2F%2Focgis.com%2Farcpub%2Frest%2Fservices&arcgis_online_token=&timeout=9000&select_folder=383&select_layer=0&select_folder_text=City_Boundaries+%3Csup%3EMapServer%3C%2Fsup%3E&select_layer_text=0+%26%23x21E2%3B+City+Boundaries+%3Csup%3EFeature+Layer%3Csub%3E+esriGeometryPolygon%3C%2Fsub%3E%3C%2Fsup%3E
Step 5.
los angeles county building foot print is here
https://transparentgov.net/json2tree/esri/server/folder2.html?org=https%3A%2F%2Fpublic.gis.lacounty.gov%2Fpublic%2Frest%2Fservices&url=https%3A%2F%2Fpublic.gis.lacounty.gov%2Fpublic%2Frest%2Fservices&arcgis_online_token=&timeout=9000&select_folder=28&select_layer=0&select_folder_text=LACounty_StreetMap+%3Csup%3EMapServer%3C%2Fsup%3E&select_layer_text=0+%26%23x21E2%3B+Cities+and+Communities+%3Csup%3EFeature+Layer%3Csub%3E+esriGeometryPolygon%3C%2Fsub%3E%3C%2Fsup%3E