This project really showcases the power of open source and passionate people building something for the sheer joy of it :)
It's basically an EP32 chip with a tiny smidge of custom hardware that's been programmed to speak the serial protocol of quite a number of 8 bit machines.
I have one for my 800XL and that speaks Atari's SIO protocol.
The depth and breadth of software for the thing is amazing, and overall I find the whole project incredibly inspirational.
Lately, they've been on a kick of creating a project where they've instrumented classic Atari games to post high scores on the internet, with a website 'lobby' where you can sign up to play games online with others.
Totally love mine, and which I had a bigger house so I could have an Apple II and a C64 and get the Fujinet for those platforms as well :)
I have an ESP32 set up with Zimodem which just makes the ESP32 act like a Hayes modem that talks IP instead of phone numbers to the serial port. They're a ton of fun.
FujiNet appears to offer a little bit more in the way of high level service translation.
considerably more. :)