DNS Server Priority (or: Is there a Primary/Fallback DNS)

I added 2 DNS Servers in the Tailscale DNS Settings.
I’d like to know if the nodes use them both (alternating/random) or only use the second one if the first one isn’t reachable from the device.
My nodes are 1 iPad, 2 Android phones, 1 Raspberry Pi with raspbian on it, and 1 Windows 11 laptop.