What do you want to achieve? Keep it simple and clear!
I want to turn rbxthumb:// into https:// (or http://).
What is the issue? Include screenshots / videos if possible!
There is no issue.
What solutions have you tried so far? Did you look for solutions on the Developer Hub?
I didn’t find anything at all anywhere.
Is it even possible?
Not directly but you can request it through the thumbnails API. The v1/assets endpoint will give you the URL for a thumbnail for an asset (ie a mesh, decal, catalogue item etc…)
https://thumbnails.roblox.com/docs/index.html
Inside of the response, look at the field imageUrl, it will be an URL that points to an actual image (you can paste it in your browser to try yourself).
As for avatar thumbnails, you can look under the endpoints under the Avatar section.