Updating SQL Server 2016SP2 to latest CU in Cluster...what happens?
Install the CU on a passive node, failover, install on another passive node, repeat until all nodes are patched.
When you failover to the passive node, the databases will be upgraded. If you failback, the databases will be downgraded back to the old CU.  (This is true for CU's, not for SP's or Major Version upgrades).