For the Preferred Site, a primary node is selected from one of the nodes on this site. Selections for the Non-Preferred Site are similar, so long as there are nodes available on the Non-Preferred Site.

The vSAN primary node and the vSAN backup node send heartbeats every second. If communication is lost for five consecutive heartbeats (five seconds) between the primary node and the Backup due to an issue with the Backup node, the primary node
chooses a different ESXi host as a Backup on the remote site.

This is repeated until all hosts on the remote site are checked. If there is a complete site failure, the primary node selects a Backup node from the Preferred Site.
A similar scenario arises when the primary node has a failure.

When a node rejoins an empty site after a complete site failure, either the primary node (in the case of the node joining the Preferred Site) or the Backup (in the case where the node is joining the Non-Preferred Site) will migrate to that site.
If communication is lost for five consecutive heartbeats (five seconds) between the primary node and the vSAN witness host, the vSAN witness host is deemed to have failed. If the vSAN witness host has permanently failed, a new vSAN witness host can be configured and added to the cluster.