VMs should for best performance be on isolated cores (but entirely up to you). If @SpaceInvaderOne suggested to put a docker container onto isolated cores, then that's a mistake. In particular the screen shot at 8:46 is incorrect
The OS will run on cores 0 and 4, and plex will run on 0, 4, 1. It will never run on 5 because both 4 and 5 are isolated from the OS. And with multiple isolated cores pinned to a container it will only ever run on the lowest isolated core.
FCP is correct.