Apple fused a high performance core to a platform which used to run a latency critical platform, the iPhone. Actually it happened over the years.
- They learned to design a phone platform.
- Then, shoehorned a modified version of macOS on top of it.
- Then, they started to iterate on performance. Added iPad. They increased the performance while keeping latency criticality of iOS platform. Trying to have performing CPU/GPU and trying to keep power consumption low paved the way, year over year.
This iteration over and over resulted in M1. A descendant of iPhone and iPad, but with unbelievable performance. They kept the best parts of every platform since they can design from the processor to the latest bit of the software.
At the end of the day, M1 is greater than sum of its parts. An M1 Air behaves a lot of like an iOS device in some parts. Connect to power, screen comes on, a big battery appears with "XY% charged" message and you hear iOS charging chime. It's funny, yet hints on the heritage.
Apple fused everything together. A unified ARM platform. Variety of OSes with the same roots. One optimization for all. With a big blow, they theoretically reduced the software work they have to do dramatically, so they can build magical software again with last effort and can transfer technologies from one to another virtually with no effort.
See: iPad Pro with M1. iMac with M1, MacBook Air and MacBook Pro with M1. They are the same device. Same performance. A slightly different OS on iPad. That's just it.