Google usernames - When you sign up for a Google Account, you choose a Google username. You use your Google username to sign into all Google services (e.g. Docs, Blogger, Gmail, YouTube, etc.). It also becomes your Gmail email address. If your Google username is BettySue1975, your Gmail address will be BettySue1975@gmail.com. Google usernames are used to sign in on YouTube.
YouTube usernames - You only have a YouTube username if you have a channel that doesn't use a Google identity. A YouTube username identifies you to other members of the YouTube community when you do things like upload or comment.
Google and YouTube usernames are separate. Since your Google username and YouTube usernames are separate, you might not be able to choose the same username for both. So even if you’re BettySue1975@gmail.com, BettySue1975 on YouTube might not be available.