How can I see which contacts have been assigned which roles and permissions?
Welcome to SE! The place to start is in the documentation about roles and permissions at https://docs.civicrm.org/user/en/latest/initial-set-up/permissions-and-access-control/

It does depend on the CMS as Jaap says, but they are all covered here. You will see who is assigned to which role in the CMS (Its under Users in WordPress) but then you need to allocate permisssions to these rolls to control which CiviCRM features each role has. There are some more complex option too, with roles and permissions specific to CiviCRM, but you probably don't need these.