Why can I boot from a USB device without "making it bootable"?

It seems like you can boot from a USB device without a bootloader to me. Even my 6 years old computers that still have a BIOS.

From what I read the BIOS simply looks for an mbr, loads it and executes it. But I have never knowingly written an mbr to any of my USB devices. So how is it that I can still boot from them when certain files are present? I doubt that the mbr could be inside of a file, because how would you ensure that it is exactly at the beginning of the partition.

So what am I missing?

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