It's not an open server that anyone can just connect to, only specific Plex accounts that you invite.

They also don't need to know anything about how to connect to your server, it simply shows up as a server name in the list on their "app" or the Plex website, there is no need for dynamic DNS or VPN or anything like that as Plex's servers do a kind of "service discovery" automatically.

It does require you exposing the Plex TCP port to the internet, so you are relying on them making sure there are no vulnerabilities. You could probably do a VPN approach and give them a URL they can enter into a web-browser, but it's unlikely to work on TV and mobile apps.

I've given access to my Plex server to my siblings and parents and it's great. Since I got a decently fast fibre line, no more having to manually copy shows/movies they want to watch over a LAN or using a USB HDD or memory stick.