devs have to eat too, open source software (when fully free) is sometimes built as a hobby like Jellyfin, or as a portfolio project, or worst case scenario as a bait and switch to paid (sometimes it's death sentence). Then there are sponsored projects like Vulkan, or others that live off of donations like Mozilla.
I always see this but I used Plex on my iPhone for months before I bought the Plex pass. I did buy the Plex app way back when and maybe that’s why but I’m also using a new Plex account so I don’t see how’d they be tied together.
I'm not sure if it is exactly the same on Iphone. But on Android you can choose to either pay for the app( a one time payment) or have Plex pass. Either way lets you watch on the mobile app.