Is Multi-Boot Possible in Android? Can you replace the bootloader of Android to create a UEFI-like environment where you can Multi-boot into another Operating System? I've seen several off-brand phones do this but I'm unsure how they do it. There is a phone on the market
(I think on Amazon) where users report that their phones don't directly boot into android but give them a choice of booting into "Android Core" and "Android OS". If you can actually do this, is there a way of flashing this ROM onto a phone, or are there other steps needed to accomplish this task?
Yes it is possible. As Irfan mentioned there are hacks which allow, essentially, dual booting with a Linux distro, the list of which is rapidly expanding.
Dual booting native Android ROMs is possible as well. Dual Boot Patcher app by xda developer Anik_khan allows just such a scheme. Additionally devices with A/B partitions certainly can dual boot(although /data is shared). It is also possible to configure a ISO within an  Android device that can be bootable via USB, say for a laptop...