The project is called "Tactility" and its website is https://tactility.one/
You can run ELF binary apps directly from an SD card without restarting or flashing the ROM. There's an SDK for building these apps, but I haven't made an official release yet.
I wrote a blog post with some background information: https://bytewelder.com/posts/2025/01/06/tactility-one-year-later.html
Source code and project files: https://github.com/ByteWelder/Tactility
I don't expect regular Python apps to ever run on Tactility, as the memory requirements are likely too high. There is at most about 8MB of RAM for apps available. Perhaps a special build of micropython could work at some point.
You can open the Files app and there's a folder named "sdcard". So far, all SD card implementations are done via a serial interface (SPI). I cannot mount them on-the-fly yet - only at boot.