There are two types of group in the SharePoint online: Microsoft 365 group and SharePoint group.
Microsoft 365 group has two memberships, owner and member. And SharePoint group has three memberships, owner, member and visitor.
When you create a new Team site, a Microsoft 365 group and three SharePoint groups are also automatically created with Team site.
By default, three SharePoint groups have permissions for Team site. But the Microsoft 365 group do not have permissions for Team site. So, you need to add the Microsoft 365 group into Site members.
The difference between Microsoft 365 group and SharePoint group is Microsoft 365 group can used in the SharePoint, Teams, Exchange and etc. Microsoft products, but SharePoint group can only be used in the SharePoint.
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