They just recently released their toolchain for the reMarkable 2 https://github.com/reMarkable/linux/issues/7
The toolchain for the rM1 has been out for a long time. Since it's just a linux device, there is a ton of existing documentation on how to work with it.
unfortunately, the sdk doesn't work with hi applications yet. the way the screen works has changed significantly. There are community projects that make it work though: https://github.com/ddvk/remarkable2-framebuffer