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 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