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Dear LS321,
Thank you for reaching out to the Microsoft community. We are delighted to offer our assistance.
From what I described to you in another post, you can try a little test by adding users as Owner in two different ways, so when you click on the Owner one in People and Groups, which users can you see? Only the users added by Share site only. So where are the other owners? Put your cursor on the first group and you will see the left users in the Microsoft365 group:
This section is a bit confusing, so you can take a look at these presentations below that will help you understand the difference between Microsoft 365 Groups and SharePoint Groups faster.
Have a great day.
Sincerely,
Sean | Microsoft Community Moderator