Should I put users in admin DB or in the DB that they should have access to
in general not just Mongo talking in general given root/sysadmin privileges or adding all users to the admin groups is not recommended due to obvious reasons. And in mongo like advanced databases there are enriched security groups and roles can be created and assign each user as per the requirement. please refer the mongo documentation on creating user roles and assigning privileges.
https://docs.mongodb.com/manual/tutorial/manage-users-and-roles/