YouTube will begin offering usernames to make it easier for community members to find and interact with each other. Each channel username is unique. Others will be able to use it to mention you in comments, community tab posts, etc.
|Over the next few weeks, YouTube is gradually offering all channels the ability to choose a username. When can I do it? You will receive another email and notification in YouTube Studio. under most circumstances, If your channel already has a custom URL, it will be reserved for you to use as your username. If you don’t like the username reserved for you, you can change it. Additionally, the platform says you can choose a username for your channel even if you don’t have a custom URL yet.|
|Starting November 14, 2022, if you haven’t chosen a username for your channel, YouTube will automatically assign you one, which you can change in YouTube Studio if you wish.|
what you should know
A YouTube username is a new way for people to find and connect with you. Unlike channel names, usernames are unique to each creator, making it easier to get noticed on YouTube!
Your new username will become part of your channel URL. In most cases, your custom URL will be your username. When viewers are not on YouTube, you can use your username to direct them to your channel. For example, if your username is @user123, your channel URL would be https://youtube.com/@user123.
More details here