iOS doesn't magically need less RAM. It just aggressively kills background apps to stay within memory limits, then makes you wait while it reloads them from scratch (burning battery with CPU cycles and NVMe reads instead).
> RAM uses electricity and costs money so it makes perfect sense to pack less of it
No, it's bullshit. Apple kept RAM low to save money, not battery. They could easily pack 16gb with negligible effect on battery life.