I don't specifically know, but typical NAT can use stateful flow information, i.e. "a UDP packet originated from inside the NAT to external IP/port X, packets f

I don't specifically know, but typical NAT can use stateful flow information, i.e. "a UDP packet originated from inside the NAT to external IP/port X, packets from that external IP/port should be routed to the originating NAT/customer" (see https://en.m.wikipedia.org/wiki/UDP_hole_punching https://en.m.wikipedia.org/wiki/UDP_hole_punching)