How do different operating systems interact with each other on the same computer?

Question

Windows

What will happen to the bootloader if you try to install:
1) Windows 10 x64 and Windows 7x32 on the same GPT disk?
2) the same, but on an MBR disk?
3) the same, but on different disks (MBR for seven and GPT for ten)?

It seems like when installing a second OS, it will write its MBR record to the zero sector. How will it be implemented in these three cases? Whose bootloader will be registered, which system will stop booting and why?

Answer 1

If you install Win 7 first, and then Win 10 with a standard disk layout (where the bootloader is on a separate partition), then both with the first option (UEFI / GPT boot) and the second (Legacy / MBR) everything will be fine and at boot there will be a choice of which load OS.
Regarding the third option, I won’t say exactly how it will be, but you definitely can’t do without dancing with a tambourine.

Answer 2

When installing several versions of Windows on one disk, the rule always applies - install from old to new. The loader of newer versions picks up the settings of the already installed one and adds it to the boot menu itself.

Answer 3

It seems like when installing a second OS, it will write its MBR record to the zero sector. How will it be implemented in these three cases? Whose bootloader will be registered, which system will stop booting and why?

All modern bootloaders can create a menu with a choice of OS, and the bootloader of a specific OS is loaded not from the MBR, but from the boot partition of the system partition, or even directly from the boot file.
A newer OS, usually knowing about older versions, will correctly detect previous versions of Windows and add them to the menu. An older one may not do it correctly. But in principle, you can fix bcd with your hands