If the switch supports it, you login with local credentials first, navigate to its config page and configure LDAP under there. You’ll tell it the IP address of the LDAP server as well as give it its client side configuration. You give it a bind account credentials (a dedicated service account with as minimal permissions as needed) that it uses to lookup the users on the server as well as Organization Unit paths and such
When a user goes to login the switch will query the provided credentials against the LDAP server, if it’s valid the LDAP server will respond with a success and the switch will log the user in
Generally there is always a local account fallback in the event that the LDAP server is unavailable for whatever reason