Earlier this year, I committed to mapping my city (approx. 175,000 people) and I worked on it for about a month or so. That's where the bulk of those came from.
Using Rapid editor, it was much faster to get buildings done, but only if they were identified properly. Many were not, or needed to be done manually, especially new developments (of which, we have a lot of).
It was honestly a blast to do, and I've already started added buildings to a neighboring municipality 😁
My main focus has been cycling infrastructure, bike parking, and small POIs to help people who aren't driving.
In Belgium, we are running a slow campaign where we are importing the official building dataset. Most mappers import a bit of the buildings and do a general update as well. (BTW: we have permission to use this data and did a thorough discussion with the local community, so the import guidelines are followed)