The oldest account have a direct URL in the form of youtube.com/username. Newer account had youtube.com/user/username (I think most account have both URLs).
Then YouTube was bought by Google and they introduced "channels", so some channels had youtube.com/channel/username.
Then YouTube wanted to become like TikTok and they forced at-usernames some years ago, so now accounts have a URL like youtube.com/@username
The ~ character for home directories was an old convention that dates from the ADM-3A (1976) terminal used by some early Unix users. The keyboard on that terminal happened to have the cursor control word "Home" on the "~" key. This shorthand was adopted by shells like sh/csh and emerged in HTTP urls as /~user/ being the shorthand for a user's personal web page on a site.
Much later in history Twitter popularized the form "@user" to refer to a personal identity. I'm not sure if they invented the usage or not. This is distinct, but probably somehow cognitively related, to the use of "user@host" for email addresses after bang paths fell out of favor.
For reasons I can't quite put my finger on @user seems a much better sigil than ~user to me, so I'm not bothered that it's become popular.
It makes sense in a chatroom if you direct a message @someone (at someone), or if you direct a tweet @someone. So I guess the natural progression of that is @someone becoming the identifier.
> For reasons I can't quite put my finger on @user seems a much better sigil than ~user to me, so I'm not bothered that it's become popular.
I think this makes sense if you pronounce the action. On Twitter you'd tweet [at] user(s). I think it made even more sense back in the Twitter via SMS where you had to send a message to Twitter's number but direct at a particular user.
Wikipedia claims it was invented as an ad-hoc convention by some Twitter users, and eventually it became so popular Twitter started turning @username into links.
I remember signing up for an AI inference provider, noticing my profile page was "/<username>" and changing my name to logout which worked but caused some amusing errors. Oops!
GitHub manage to do it. Most URLs you'd think of are either redirects to other bits of the site, or accounts owned by GitHub themselves. It just takes a bit of planning.
You can plan every "top level" path you'll ever want on the site from now until forever? Or do you mean planning as in plan to force account name changes on users when someone's username conflicts with a new feature name?
I would be very confused if flickr.com/contact went to a user page.