I'm trying to understand its github if holepunch itself open-source or not. Hypercore is but is holepunch different?

AFAIK, "Holepunch" is the overall project which is all open source. It is composed of https://docs.pears.com/ , https://github.com/holepunchto/libpear which runs on https://github.com/holepunchto/bare/ which runs on a V8 interface to libuv.

And, then Hypercore, Hyperbee, Hyperdrive, Autobase, etc are all modules you need to get things done so you can make an app like their serverless Zoom clone https://keet.io/