You need to flash an official raspberry Pi OS image to a SD Card
You can use the official raspberry Pi Imager
sudo apt install rpi-imager
Imager will create 2 partitions on your SD Card.
The first partition is a fat32 partition called system-boot. It contains the kernel, devicetree and bootloader.
You need to delete all files and place your
The second partition is an ext4 partition called writable. It contains the root filesystem
You boot RaspiOS like any other distro on Surface RT.
There is one thing to do after you can see the desktop.
Open the Terminal and enter
Now your SD Card is writable and can use all of the space.