How can I get polygons of all cities, regions, countries over the world? May be there some API query using the Overpass API? Or there are some database? |
https://wambachers-osm.website/boundaries/ allows you to download admin boundaries. I don't know whether it is the best way to download all admin boundaries. |
If you don't need the full detail of OSM boundaries you could also look at https://www.naturalearthdata.com/ for country and region boundaries. |