How does Apple build the RAM into their M chips? Why don't other manufacturers do this?

Another reason why it works for Apple, IMO, is, because of their vertical integration of chip and device, they don’t have to produce the wide variety of chips like Intel and AMD do in order to meet the diverse needs of their customers. For example, the M3 comes in base, Pro and Max models, and further for example the M3 Pro only comes in, I think, 18 and 36 GB variants. Between M2, M3, and M4, Apple makes, like, only a couple dozen different SoCs. Compare that to Intel who has 42 different Core Ultra Series 2 processors, and they could be paired with anything from 8 GB to 128 GB or more depending on what Dell, Lenovo, or an enthusiast wants to do with the system.