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...
If you are talking of having multiboot solutions on the latest Android 9 and Android 10 versions ... that doesn't see to be possible currently and there isn't much interest among the dev community in developing these systems for these newer android versions as android from these versions onward's has undergone some fundamental changes.
But for older versions you do have solutions like ...
- multi boot patcher
but all of the above are pretty much limited to the devices they support. So you need to do some research to identify if your devices are supported on any of these implmentation's.