Plex is freemium in the sense that the software walls off certain features. But the content is always hosted by you; Plex doesn’t actually control any of your media. You’re not hosting it on Plex’s servers. So there’s no way for them to realistically take your media away from you.
Jellyfin is the FOSS alternative, but isn’t quite mature enough to overcome the “friends and family factor” in many peoples’ setups. Basically, Plex makes remote access easy, by proxying the initial connection. The same way DDNS turns a dynamic IP address into a static URL. You host the media server, it tells Plex “hey, I’m located at this address”, and then your various devices simply ask Plex what your server address is. So in that sense, connecting with plex “just works” much easier.
Plex also handles all of the signup and onboarding stuff; Sharing your media libraries is as simple as having them create a Plex account, sending each other friend requests, then granting access to your server. Plex handles all of the backend authentication stuff, and they made their own account, so you’re not bogged down by managing a ton of different accounts.
With Jellyfin, all of that DDNS and account management stuff needs to be set up by you. The Jellyfin devs don’t host any centralized servers, so there’s no way for an app to ask what your server’s address is. And since they’re signing in directly to your server, (because again, no centralized service to handle that), it means you as the server owner are managing accounts for all of your friends and family. So if grandma forgot her password for the fifth time this month, you’re the one resetting her password.
There are other ancillary services that help smooth a lot of that out, but setting them up is a fairly obtuse process if you’ve never done anything like it before. And it also means that you’re setting up six or seven different containers, just to emulate what Plex does right out of the box. The demographics here tend to skew towards system admins and IT professionals, so all of the inevitable “psh, setting it up is easy. I did it in 15 minutes” comments are built upon a lot of external knowledge and experience. Of course, there is a relevant xkcd for that:
On the bright side, you can run both in parallel, and see which one you prefer. They’re just providing access to existing media folders (and indexing that media to grab metadata, album art, etc,) so they’re perfectly fine to run side-by-side. Many people (myself included) do so.