Why does the PC prompt me to boot from a USB flash drive in normal (non-UEFI) mode?

Question

BIOS

There is a flash drive - it has an MBR disk with one active partition, type ef and a FAT-32 FS rolled onto it, on which the contents of the Windows 10 ISO image are written. In fact, yes, the system prompts me to boot from this flash drive in UEFI mode by default. But there is also an option to boot in Legacy mode. But it's not logical. Legacy mode requires that a first level bootloader be written to the MBR on the flash drive, which will call the second level bootloader. Why then does it work? After all, I just unpacked the files, I didn’t write anything to the MBR.

Answer 1

Why then does it work? After all, I just unpacked the files, I didn’t write anything to the MBR.

Because there is a boot sector at the beginning of the bootable ISO image. See https://en.wikipedia.org/wiki/El_Torito_(CD-ROM_st...