Plex does operate a few services that 'free' (or any) streaming may rely on depending on circumstances. Aside from auth, Plex clients that aren't able to discover the server instance locally rely on a Plex-hosted webservice to enumerate available servers. Additionally, there is a client-side config option that allows low resolution streams (720p is the ceiling, I believe) to be proxied via Plex's infrastructure. This setting is referred to as 'indirect connection', if I recall correctly.
magicalhippo|11 months ago
That is, your host gets a DNS entry along the lines of 1.2.3.4.something.plex.com, where 1.2.3.4 is your Plex server's public IP, and they have a deal with a cert issuer so they can get valid certs for that hostname.
edit: It's Let's Encrypt these days[1], was sure it wasn't that when it started, but it's been a while.
[1]: https://support.plex.tv/articles/206225077-how-to-use-secure...