Here I show briefly how to add apartment buildings in OpenStreetMap using the web-based editor on https://www.openstreetmap.org/ and also the desktop Java editor, JOSM. Installing and setting up JOSM is not covered.
Mapping local apartment buildings was useful to support spatial analysis about Neighborhood Associations and Housing Equity
In the video I have already gone into "Data... Set Building Size" and set the buildings I draw to default to having the building=apartments tag.
The tag for an apartment building is "building=apartments".