How does Sega Genesis (Mega Drive) Multicarts work?

[ZetTheLegendaryHero]

Searching Sega Mega Drive (bootleggers like that name even if the actual cart is shaped like a Genesis one) on Aliexpress under Memory Cards under Video Games shows that pirate and bootleg multicarts are still being produced for the Sega Genesis (or Sega Genesis clones to be specific). Also during the Sega Genesis's own life time (before after market Genesis consoles and games) there were some official multicarts on the system.

How does Sega Genesis multicarts even work? Are they basically non-rewritable flash carts? Also, how come Sega Genesis pirates and bootleggers can seem to make multicarts just as easily as regular carts? Finally, is it possible for someone to make their own Genesis multicart in their garage, without the use of a rewritable flash cart?

I'm guessing one answer to the last one might involve the use of a FPGA (although I'm probably wrong).

By the way, since I'm already posting, is it possible for someone to make a repro cart of the Sega Genesis version of Virtua Racing? What about in his or hers garage?

Again, I'm assuming it might involve the use of a FPGA (though I'm probably wrong again).

Anyway, these are probably stupid questions, but I never really understood multicarts (I'm more of a software guy than a hardware guy).

[Flojomojo]

I assume things like the 18-in-1 are like the old 6-in-1 cartridge, a bunch of ROMs compiled into one runtime with a menu front-end, non-rewritable flash as you say.

 

I have a "600-in-1" on the way that is branded as an Everdrive. I assume it works in a similar way to the "real" Everdrive, where the front end is on the cartridge but the ROMs are on a removable SD card.

 

You'd need the SVP math chips to roll your own Virtua Racing. I guess you could do it in FPGA but as I write this that would be prohibitively expensive.