Firmware that runs on the ARM9 core.
The compiled .bin file should be loaded somewhere accessible to the ARM9 processor and executed starting from the first byte, it'll relocate itself as necessary.
The 3DS has multiple ARM11 cores that are the main application processor which for linux-3ds run the kernel and userspace. The ARM9 core gets loaded with this firmware. It communicates with the ARM11 cores via the PXI protocol, and facilitates SDMMC access (though controller select can be transferred to arm11).
GPLv2