Updating SQL Server 2016SP2 to latest CU in Cluster...what happens? Updating to latest CU in a cluster with a bunch of databases.
Can I do this on the passive node without SQL Server kicking on?  Do I need to worry about that?
I think I can run it on the passive, reboot the passive, then failover and ... at this point I'm unsure what happens with the databases in the instance.
Does it go through and update each individual database?
Run update exe on passive node
Reboot passive node
Failover...?
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).