Notesnook is not open source (you cannot self-host it, you must pay them). FWIW, Standard Notes is also very difficult to self host, though if you dig deep enough you can actually find the server and some outdated documentation.
That is a half-truth at best. Maybe the applications (viewers) are open source, but not the server (the thing that actually stores your notes). So you cannot self host it, which is what OP was talking about.
FWIW, I settled on https://github.com/zadam/trilium which is very bare-bones but fully open source and self-hostable.