Apple's RAM is on-package, not on-die. The memory is still a separate die which they buy from the same suppliers as everyone else.

https://upload.wikimedia.org/wikipedia/commons/d/df/Mac_Mini...

That whole square is the M1 package, Apple's custom die is under the heatspreader on the left, and the two blocks on the right are LPDDR packages stacked on top of the main package.

https://wccftech.com/apple-m2-ultra-soc-delidded-package-siz...

Scaled up, the M2 Ultra is the same deal just with two compute dies and 8 separate memory packages.