How to estimate building height

Perhaps someone with more knowledge about OSM Buildings' implementation can add some information here, but 3D renderers often use the same default height for all buildings without height or building:levels tags (perhaps with some random variation thrown in). And from the looks of it, OSM Buildings is no different in that regard.

If you want to go down that road, one resource that you could use to guess heights is the value of the building key. While most buildings use yes as the building value, quite a few have more detailed tagging. When estimating heights, an apartments building is probably taller than a shed or garages building. There are other tags that might be found on buildings, such as shop, amenity or even man_made=tower – all of which allow some refining of your initial estimates.

Ultimately, though, I doubt that you can produce sufficiently reliable estimates on that basis. Keep in mind that, even though these tags are a lot more common than level counts or height measurements, they are still only available for a minority of buildings. And even where they exist, there are exceptions for all assumptions one might make about "normal" heights for a particular building category.