I have been using Plex for quite some time already, with its media server running off of a Raspberry Pi 4 (RPi4). I use it to watch videos on the Apple TV with the Plex app - all free. Plex has a paid subscription tier, PlexPass, that provides more features, most of which I don't need.
Until recently, I went looking for a free, open source software (FOSS) alternative to Plex, and found Emby and Jellyfin to be popular options. Between the two, I decided to go with Jellyfin.
Installing Jellyfin via Docker was uneventful. One thing about Jellyfin, it has support for third-party plug-ins, which IMHO Plex does not have. I have yet to maximize the plug-ins, but I have installed Open Subtitles, Trakt, InfuseSync, and Webhook. 
Jellyfin server interface c/o jellyfin.org
The RPI4 seems to be pretty capable of running both Plex media server and Jellyfin at the same time. Tried watching, using Infuse, on the Mac and the iPhone at the same time and Jellyfin did not hiccup. Kudos to RPi4!
Jellyfin comes with official applications for iOS/iPadOS, macOS and Apple tvOS, but I have decided to use a third-party application, Infuse. Infuse supports Jellyfin, Plex and Emby, along with viewing media from other sources, unlike the dedicated official apps, like Plex and JellyFin Mobile/SwiftFin. Infuse is free, but there is a PRO version that adds support for higher quality video and audio formats (think Dolby Vision and Atmos), device syncing, and streaming from cloud services, and even Spatial Audio!
Plex and Jellyfin offer the same functionality that I need, i.e. watching videos locally, the FOSS nature of Jellyfin is huge advantage, not to forget the privacy it brings. Right now, I am leaning towards using Jellyfin + Infuse, but we will see in the next few weeks.