These plugins made available by Electrum, I even tested one of them a while ago, it'd be interesting if someone paid attention to producing wonderful possibilities to add more features to Electrum (which is already an incredible wallet).
I know about the built-in plugins, yeah, but those are bundled with the Electrum package so nobody has to actually think about them being setup or even know that they exist, as they might as well be features in the main Electrum program.
There is an interesting plugin (I've never tested it and I don't even know if it's still maintained) BTW which basically makes it possible to connect to the full node (similar to belcher's personal server).
How is that different from just using an Electrum personal server, like Fulcrum or ElectrumX or something like that? It is more actively supported though.