https://www.macobserver.com/analysis/understanding-apples-un... is an article which describes the architecture. It's not on-die, it's next to the CPU, basically as close as it could physically possibly be without being on the die.
Here's a photo of an M1 CPU:
https://www.techinreal.com/wp-content/uploads/2021/05/2feddf...
Those chips on the right side are LPDDR4x chips (which you can verify by googling the part numbers visible on them). They are "off-the-shelf" so to speak, not custom on-die memory.
