I'm not good with details on Amiga, but I think the procedure is:
You boot some sort of Kickstart/Workbench, then run an AmigaOS program which is the Linux bootloader and pass it the kernel and, if needed, the initrd from the AmigaOS filesystem, it will load them and make them usable, then jump into Linux. From then onwards, that one will be the OS in charge, making ext4 available etc.
Sadly, no kexec yet. Having to copy out the kernel instead of being able to load it directly from ext4 (or whatever you choose) would be cool.
AFAIHH some of the emulation projects have made available Free (as in Freedom) ROMs for TOS (EmuTOS) and Kickstart, which contain enough code to run this without needing the proprietary Amiga stuff. But, like I said, I'm nowhere near knowledgeable about this part of those architectures, plus I mostly worked on (emulated and a few real) Ataris, not Amigas, while doing this. (And even there, I did as few as possible on the "native" side.)