There is no such list.
There are several data centers around the globe (often Amazon AWS) which host part of the infrastructure which makes plex.tv “tick”.
It lies in the nature of cloud computing with dynamically added and removed ‘nodes’ for load-balancing, that there are no fixed IP addresses.
And yes, clients communicate (preferably) directly with the server. So mobile clients usually have a different IP each time.
Normally, plex.tv only serves as a wayfinder for clients to the server. (kinda like DynDNS, with added TLS certificates)
Only if the clients cannot reach the server directly, they try to use a Relay connection which again uses AWS data centers.
