Where can I get the source code they modified?

That comes with a big caveat. You can either choose to use the printer offline, or online, with no ability to use both. If you want the ability to monitor or pause a print when you're not home on the off chance something goes wrong, you HAVE to send every print through their cloud, there's no middle ground.

That's not Bambu being open, that's them doing the absolute minimum to allow people to say "you can use Bambu printers fully offline" in comment sections.