I've tried researching this before, but was unable to encounter an answer. Can you boot a rooted device through USB-C, SD, or something? I'm currently running a Galaxy S9+ and want to keep Android Pie on the internal storage, but I want to choose to boot from an external device through the bootloader (if there is one). I know how to access recovery mode and such (the mode where you hold the Bixby button, volume up button, and power button), but I don't know if a bootloader can be accessed. Please inform me if it is possible to boot from external hardware via a bootloader, not sure how strict Google is.
You always have to modify boot.img, so the answer is no. If you considering to edit boot.img and manage to mount the external disk to /system, you should mount another partition to /data too.
There are apps like Multirom out there which can loop mount ext4 image files located on internal storage. Afaik not for pie, you should try with some old device first before soft bricking your S9+
mmcblk0withmmcblk1. This includes - but not limited to - modification of RAM disk (like init.rc) files insideboot.img. For that, bootloader needs to be unlocked. On Treble devices, you will also need to modify device tree blob (dtb) which is usually appended to kernel image, also insideboot.img. Kernel cmdline fromboot.imgandfstabfile from/vendorpartition also need to be modified.