That’s just an APU, see consoles and laptops. The unified memory is basically just the above, but Apple also claims that due to Apple Silicon having the storage controller on board, the swap is magically faster 🤷
Also Mac OS/Linux use less RAM than Windows which certainly helps.
8GB is “fine™” on a MacBook Air, but it’s criminal for a Pro machine, and it certainly should not cost £200 for an extra 8GB. That’s genuinely insane pricing