When you create a modern team site, it creates owner, member, visitors where you can add by clicking on the cog wheel, site permission and add member and then promote it to owner. for visitors you can share without edit and the user will show up in visitors. However, if you click advance permission, you fill that none of the users feature in members, owners, visitors. This makes me think that the containers are different. So, my question is how they are related. how do we go about it as for any document library or list where unique permission is required.
Secondly when you click Access request settings in the ribbon, you see sitename<admin> as a group to receive access request. "Who are the members of the group.
Security model in modern team sites is a little bit different to what you have in classic sites. Basically, access to the site is governed by the Group you have attached to your modern team site: (1) Group Owners are indeed site collections admin in the underlying Group site (2) Group Members are added to the defalt SharePoint Site Members Group
Beside this difference, you still have the "old" SharePoint Groups in your site and you can use them to configure security at a more granular level