The Modern Team sites provide a collaboration environment for your teams and projects. Each team site, by default, is part of a Microsoft 365 group, which includes a mailbox, shared calendar, and other collaboration tools. Team sites may also be part of a team in Microsoft Teams. Permissions for team sites are best managed through the associated Microsoft 365 group or Teams team.
Team site permissions and Microsoft 365 Groups
https://docs.microsoft.com/en-us/sharepoint/modern-experience-sharing-permissions#team-site-permissi...
By default, each SharePoint team site is part of an Microsoft 365 group. a Microsoft 365 group is a single permissions group that is associated with various Microsoft 365 services. This includes a SharePoint site, an instance of Planner, a mailbox, a shared calendar, and others.
When you add owners or members to the Microsoft 365 group, they're given access to the SharePoint site along with the other group-connected services. Group owners become site owners, and group members become site members.
It's possible to manage SharePoint site permissions separately from the Microsoft 365 group by using SharePoint groups, but we recommend against it. In such a case, group members will continue to have access to the site, but users added directly to the site won't have access to any of the group services. The exception is view-only access - Microsoft 365 groups don't have a visitors permission for view-only access, so any users you wish to have view permissions on the site must be added directly to the visitors group on the site.
