Based on the MIST Project, the Mister program takes what was done on development boards and finalizes it on an evaluation board. Many cores have been developed for the Mister system.
 
What I am reading/seeing, one can have multiple cores (CPUs/Systems) on a single board by storing them on an SD Card and then program the core you want on the FPGA chip. I have to be wary of that because FPGA chips have a limited number of writes like an EPROM, FlashROM, CF/SD cards; but how many is writes one has is unknown.
 
Many of the Mister cores have been work on to near perfection, including Amiga, 8 Bit Commodores, many game units (Sega, Nintendo, Atari, etc), and arcade units. More cores are being added almost daily. Running in the GHz scale, there are many hacks and modifications to the hardware core programming that fixes a lot of issues of the original systems like video glitches, program slow down, unresponsive controllers, etc.
 
But the most interesting thing is the cost. An FPGA evaluation board to run a minimum set of cores will only cost you around $90. Compared to $400 - $1200 for a development board, this brings down the price greatly!
 
I will put up links to this later as I am still studying the technology.
 

Here is a video tutorial. But it (he) requires a Windows 10 PC to do the FPGA programming.
https://www.youtube.com/watch?v=OkQJ0Vc75AE
 
SmokeMonster I have been following to check out the limits and abilities of this set up. He admits that one needs to add to the FPGA with more RAM and I/O (including video adapters) to make it more playable with more options without the lag or system crashes. His last video explains a lot but you should see the "21 System Core" videos he did on this FPGA system.
 
(Last Video of the series) https://www.youtube.com/watch?v=qqngWjUTSlI
(First Video of the series) https://www.youtube.com/watch?v=CFMKFdkFEEE
 
(The FPGA board they both use and recommend; rumored to be $90 or under on Amazon) https://www.youtube.com/watch?v=_g471imXA7U

Note: FPGA can create any circuit you want, including I/O, RAM, ROM and Video. But similar to a Flash Drive, it is limited in its size and number to floating programmable gates in the array. Some FPGAs are larger than other, like the Sparta 7, Xilinix and Altera makes one of the largest ones out there, but they are not cheap.
 
Thus when such projects say that they add RAM, video or I/O to the board, they are saving space to put in as much of the core system into the FPGA and then give it more with the add-on.
 
But as-is, emulation can be done without the add-on boards and still have video, RAM, ROM and I/O Function on the FPGA directly; but only if the FPGA is large enough to handle it all. An FPGA can be large enough to handle multiple cores running all at once (great for parallel processing) if the cores are small (or the FPGA large) enough to fit inside the FPGA.
 
The FPGA makes this all possible.

I have a win 10 desktop I need to pull out of moth balls. Trying to read this on my phone is hard and my laptop is at the camper with limited net. It’s all interesting though.