> no idea why that doesn't just lead to https://github.com/eliemichel

Pretty sure it's because GitHub allows the user to put content at the root of that URL. And it's an entirely separate domain to avoid cookie stealing or whatever.

github.io is not a forge, it's static site hosting. Completely different product than github.com, but integrates with it nicely.

People use github.io to host sites that are developed on github.com or around projects hosted on github.com.