oh, geez. No, just base64, using the URL safe alphabet. (The obvious 62 characters, and "-_" for the last two.

It's called "urlsafe base64", or some variant, in the languages I work in.

> This encoding may be referred to as "base64url".

https://datatracker.ietf.org/doc/html/rfc4648#section-5

But yeah, it's not base64 followed by a urlencode. It's "just" base64-with-a-different-alphabet.