There’s even a reliable Switch port of PCSX, an original PlayStation emulator. However, older platforms, like the SNES and Nintendo DS, work well. There are certainly issues with more modern, demanding platforms (like the Dreamcast).
You can install emulators on a modified Switch and play all manner of classic games from early home consoles, handhelds, and arcade cabinets. The most obvious of these is installing software from unscrupulous sources, including pirated games. This software allows you to do things Nintendo never sanctioned. “Homebrew” is a term used to describe user-contributed software. This means, in theory, it should maintain compatibility with first-party games and software while allowing you to use software from sources other than the eShop or a cartridge. You’re running a custom version of Nintendo’s firmware. In Apple’s case, this allows you to modify and tweak the iOS operating system, install software from unknown sources, and dig around in parts of the system you were never meant to see.