Sure its a catalog of your local media served via an API, with metadata enrichment, built in transcoding (hardware support too) and multiple user support (watch history, recommendations etc).

Its not just a web app, I would say most dont use the web app directly, instead they provide clients (Android/iOS/Roku/AndroidTV/WebOS/Tizen) that are in official app stores. This is the big thing for me, being able to just install the client on your TV and not worry about it is great.

Often the clients are just wrappers of the web client with native integrations, but they work well.

Plus there are loads of 3rd party plugins to extend it, and alternate clients, Im a fan of https://github.com/jeffvli/feishin for music duties personally.

They do a bit more than this. They do streaming and transcoding for a start, that's fairly essential for a functioning video platform. They also have native apps for a range of platforms. Support subtitles, multiple audio tracks, all sorts of codecs. And then there's all the metadata stuff as well, getting blurbs, posters, ratings, categorisations, episode naming (which has so many awkward edge cases) etc all from various metadata services. Oh and they do this all with multiple users and ACLs.

Plex also then adds social features and curation. The value of that to its users is debatable, but it's additional complexity regardless.

These are pretty big projects. A founder moving on is a fairly big deal.

A friend or relative can stream the content over the internet via Plex or Jellyfin (through a Roku, or other streaming device) via a UI that is very, very easy to use. Plex is a way for my non-technical aunt to easily interact with my media server from her home three states away.

I can browse to jellyfin on my phone, ipad, laptop, TV etc watch any format (since it will transcode on the fly) and watch whatever I was watching from wherever I left it. It will also obviously know where I'm up to in any series so if I start watching something else I can go back to it at a later date.