The poster you're asking this to is probably talking about plug-ins. Plex, very long ago, supported plug-ins, but it no longer does. Plug-ins were usually for adding in support for other media scrapers (porn and anime), or even other media types, like audiobooks.

Additionally, Plex tends to revise their UI and inner workings in a way that favors everything but the core media sharing platform. They add TV stations, they mix in their streaming ad-supported channels with your search results, and push them before the friends and family stuff, making it tough to help other navigate to shared libraries.

I think, overall, Plex is a good shepherd for their product, but everyone knows the enshitificaiton process is inevitable. It's just a question of how long the timeline between "Plex is usable" and "Plex is sold to private equity and is now utter shit." I've been pleasantly surprised with the length, so far. But having an escape hatch is always a good idea, and Jellyfin seems to be nearing a parity.