The PCB design was largely based on pictures posted at Spritesmod, but with a retooled firmware and boot ROM that are easier to build.
The cart has 16MB of onboard storage. Most Vectrex games are 4-8kb, so that equates to 2,000-4,000 games. The newest Vextreme PCB revision is USB C based, and supports the bankswitching mechanism used by some homebrew.
Operation is simple: Connect via USB to any device (computer, phone, etc) and the cart is detected as a USB thumb drive. Then simply copy games to it. The games list is read internally and its menu is automatically generated like users have come to expect from modern flash-carts like EverDrives. The firmware and filesystem will even support folders soon, and more features will be added as the cart evolves, as is the nature of open-source projects. Vextreme is light-years beyond the current Vectrex flash cart offerings, and should be cheaper as well.
The cart’s design should also be a great base for designing flash carts for other slower consoles like ColecoVision, IntelliVision, etc.
Technobly is on YouTube as BDub and has several videos documenting the evolution of the cart, as well as operation and construction videos. Rattboi and Technobly both wish to give all credit to Spritesmod for the original design, and say that they simply carried it to the finish line.