To find out which server is currently the primary when deployed in single-primary mode, use the MEMBER_ROLE column in the performance_schema.replication_group_members table. For example:

mysql> SELECT MEMBER_HOST, MEMBER_ROLE FROM performance_schema.replication_group_members;
+-------------------------+-------------+
| MEMBER_HOST             | MEMBER_ROLE |
+-------------------------+-------------+
| remote1.example.com     | PRIMARY     |
| remote2.example.com     | SECONDARY   |
| remote3.example.com     | SECONDARY   |
+-------------------------+-------------+

Alternatively use the group_replication_primary_member status variable.

mysql> SHOW STATUS LIKE 'group_replication_primary_member'