I used modrinth launcher since the whole platform is open source afaik. Big fan.
That said, I stopped playing minecraft since microsoft has perverted everything I liked about it to make it a childrens game with microtransactions. They recently announced ramping up the breakneck speed of updates to make it more like a live service game which may devastate the mod community.
I since made a voxelibre server which works surprisingly well. I also maintain a minecraft inspired texture pack since I dislike visual change.
In any case. Good luck.
Yes. I just researched and found out minecraft uses its own engine based on lwjgl. It’s a little bit like whatsapp and matrix. While whatsapp uses(d?) xmpp, matrix is the protocol, not the client.