> I've hear that even these highly impressive demos are fake - they built a ton of custom code for each one they rebuild core Unity features, meaning if you wanted to replicate this for yourself, you'd be in for a ton of development.

It's sadly even worse. To replicate most of this, you need access to the C++ part of the engine source code. If you're small (e.g. less than $10 mio annual revenue), they won't even discuss pricing with you.