If you want to know some opensource game engines:
I've heard good things about amethyst (rust but never tried that myself.
Quicksilver 0.4 also seems to become promising but that will probably never be as feature rich as unity and though I like using the current version I would only recommend it if you want to learn rust + wasm.
JMonkeyEngine is something I also took a look at in the past, it uses java and seems fine. Not exceptional but also not bad.