Are There Any Differences Between YouTube username, legacy username, and ID?
Yes, these three concepts are different from each other.
The YouTube username is one of the first things you pick when creating a channel. Once you gain 100 subscribers, you may change it.
The YouTube legacy name is no longer available, yet older YouTubers have it. You can notice that these people have /user/ item with their username in their YouTube URL.
A YouTube channel ID is a unique identifier. It consists of 24 characters assigned to each page.