Just a note that in Blender 2.81 it made the rendered thumbnail when there was no camera, but now it doesn't anymore.

Note that the thumbnail is updated every time the file is saved. A workbench-style thumbnail only takes a second or two, while one with materials and textures could take a very long time... every time you hit "save". So there is some tradeoff here between quality and time. I did some experimenting with using random object colors and found them to be good previews that were very fast to create: https://developer.blender.org/D9404

Current does have separate code paths if you have a camera or not, so ideally those would create the same type and quality of thumbnail.