I'm curious why site permissions load on some sites and not others. This particular one doesn't show anybody but on another site, it shows Owners, Members and Visitor group memberships
Site permissions menu is available in modern experience, no matter what the site template is.
You are possibly looking at the menu of a site that is not supported to connect Office 365 group, for example, publishing portal. The SharePoint groups (Owners, Members, Visitors) are automatically created in User Information list but will not be listed under Site Permissions Flyout Menu.
See the comparisons below: