The Atari 2600 PlusCart is based on Robin Edwards Unocart-2600 (https://github.com/robinhedwards/UnoCart-2600) and the extensions of Christian Speckner's (DirtyHairy) fork (https://github.com/DirtyHairy/UnoCart-2600). The PlusCart has no SD-Card, but an ESP8266 to connect to a local WiFi Network and the Internet. The PlusCart downloads the ROM-files from an Server in the Internet called the "PlusStore". The way this is done is similar to the way the Unocart-2600 loads ROMs from the FAT filesystem on the SD-card, while the VCS is performing a waitroutine in his RAM. Additionally the PlusCart has one more interesting feature. It offers internet access to the ROM Developers, these functions are called "PlusROM".
Some examples for PlusROM enabled games and demos can be found at the PlusCart Highscore Club and in Gitlab examples directory