I cannot do anything on replSet when auth is enabled

I have a mongodb cluster with one replSet, 3 config servers, and 2 mongos instances. I logged into mongos using (admin, pwd). admin is given ["userAdminAnyDatabase", "dbAdminAnyDatabase", "readWriteAnyDatabase"]. It works well. but when I logged into the replSet directly. I cannot pass authentication using (admin, pwd) again. I found that mongodb culster, replset, config servers, and mogos all have admin databases. They are different. What are the difference? what are the differences when doing authentication?

Can I create a (admin, pwd) in the admin database of the replSet? Will I have trouble when I add new replSet?

Thanks, Best,