There's not really any magic to it, they just need to implement PCIe and include memory slots instead of hardwired chips on the package. There are other ARM systems that already do this.
How they're gonna solve that is the big question.
https://www.anandtech.com/show/16252/mac-mini-apple-m1-teste...
As far as HSA/unified memory architecture, PCIe 4.0 has enough bandwidth to comfortably access main memory for applications that support it (and vice versa with GPU memory), although I imagine most high-performance GPU workloads will do better primarily operating on dedicated GPU memory (especially given that a single M1 core can already saturate the memory controller on its own).
This has also not been a problem for any other unified memory system (which typically also do not include memory on-package), and I can't see any reason why it would matter.