SWI is just software interrupt. Gets trapped by supervisor mode. Then the registers can be decoded by the kernel to implement the correct syscall. Old school real mode x86 equivalent would be an INT instruction.

If you want to do straight to the hardware bare metal then you need to do something like this (https://www.rpi4os.com/part5-framebuffer/) but you’re going to have to do all the other system init work that a modern system needs. Old computers were simpler!