UEFI or Legacy Boot

I want to work on my crochet amd64 board file project this weekend. I notice both type boot partitions on the FreeBSD11 memstick installer.
I am trying to figure out the boot process.
I guess the bootloader probes the bios to determine which to use (UEFI or Legacy BIOS)?
I see several different stages to the booatloader.
Which stage picks legacy bios boot or uefi boot?
So it looks like it is stage 1 with the various boot1 files that picks the boot type? Maybe sooner? Is it BTX server that picks?
I guess the bootloader probes the bios to determine which to use (UEFI or Legacy BIOS)?

The board firmware picks which one to load, based either on the boot code it finds at the start of the disk (MBR) or the partition type at the beginning of the disk (GPT). If your board was made in the last five years or so it's either going to only use UEFI, or it's going to prefer UEFI.
Generally speaking, no the ESP (EFI System Partition) doesn't have to be the first one on the disk. But beware - there are all kinds of bad UEFI implementations out in the wild, some of them are broken.
Second question: yes, if you do a FreeBSD install, the contents of your ESP will be the same as boot1.efifat (warts and all).
I was backwards about this... So the board picks the mode. It was right in the manual after all.
That makes sense as I have a malformed UEFI bios on Advantech MIO-2261 and I can manually choose the legacy mode to boot the USB stick from the boards 'boot override' BIOS setting. By default it tries to run UEFI even with CSM enabled. It fails when memstick installer boots UEFI.

An easy indicator for me is the color of the dialog screens for bsdinstall. Dark Blue is the legacy color and Light Blue seems to be EFI. Or is it GPT that brings the light blue color to bsdinstall? Maybe this is due to vt(4) ? Why is bsdinstaller color different blues? You see my guesses.

The handbook is very well documented for booting. I don't see much about EFI though.