how do you make crossfeed actually work (trust me this is actually a good question)

I've always been fuzzy on this as well. The way I've made it work with my Astrum Heavy is by saving the side interstages of the "Simple Heavy". Those seem to be XML edited to not have the fuel cross feed setting which I'm pretty sure automatically makes it act like it should. And could be wrong but I think you have to make sure whatever is attached to it doesn't have it's fuel line active

Decouplers can be used to control the direction of fuel flow across fuel tanks. Decoupler fuel crossfeed option very easy to find.

If you want multiple fuel tanks to act as one single fuel tank, you can use the 'Fuel Line' option in the 'Advanced Part Settings'.