Apple Belgium warranty
Does Apple in Belgium require a Mac bill bought in the store to accept warranty hence can I get the alternative Mac to use for work while my own Mac is being repaired
MacBook Pro (2020 and later)
Call the store and ask. Unless somebody from Belgium happens to see your post I don't know if there are country specific requirements. I would think if your Mac purchase was registered with Apple then it would be in their warranty database and they could refer to that.
Check Your Service and Support Coverage - Apple Support
I have no idea as to "loaner" Macs. I have not seen reference to that on Apple's support pages.
