If I use windows at home (gaming), mac at work and android on my phone - how exactly are these supposed to seamlessly work together?

There are many cross-platform password managers that sync very nicely, which would solve for the machines you control - the Windows gaming machine and Android phone.

For machines you don't control, such as your employer Mac, well that's a special case. In theory you can use "FIDO Cross-Device Authentication", which is a passkey flow designed specifically for authenticating on one device using a passkey stored on a different device, and involves scanning a QR code.

I've never tried this though. Personally I tend to avoid mixing personal stuff with work stuff, so the problem rarely arises.