An "Everdrive" is like a cartridge that looks just like a NES/SNES/N64/whatever cartridge, and is put into a real, original console. So you need the original hardware for it to work. And there is no emulation whatsoever. The cartridge actually contains a modern, large memory into which you can put ROM images, and then the NES (for example) is "fed" one of the ROMs that you pick from a menu, and from that point on, it's identical to having the actual cartridge; the NES thinks it's a real cartridge and the exact same electrons beam out from the TV, with zero artifacts of any kind since it's not being emulated, but rather "run for real".
At least, that's my understanding.
What I don't understand is how it the Everdrive handles all the special chips and stuff that they often/frequently put into the cartridges to extend them in various ways. And it's not just a small few games that did this. Many of the classics actually were extended internally with special hardware. Does it really have every single chip that any game ever used as hardware inside the Everdrive? Or does it actually emulate them somehow?