How does typical home wifi routers use primary and secondary DNS on DHCP?

Does it only let the devices use the primary DNS until failure then use the secondary DNS?
or does the devices use both at the same time all the time?

For context, I have a single pi-hole set up and I want the devices to only use the pi-hole unless it failed then use public dns such as 8.8.8.8 or 1.1.1.1.