In simpler times, some years ago, it was only necessary to place the bootloader in non-volatile memory at the reset vector of the processor. NOR flash memory was common at that time and, since it can be mapped directly into the address space, it was the ideal method of storage. The following diagram shows such a configuration, with the reset vector at 0xfffffffc at the top end of an area of flash memory. The bootloader is linked so that there is a jump instruction at that location that points to the start of the bootloader code:
Boot in the old days
From that point, it can initialize the memory controller so that the main memory, the DRAM, becomes available and copies itself into DRAM. Once fully operational, the bootloader can load the kernel from flash memory into DRAM and transfer control to it.
However, once you move away from a simple linearly addressable storage medium like NOR flash, the boot sequence becomes a complex, multi-stage procedure. The details are very specific...