Grub (Eng. GRand Unified Bootloader) —loader of the operating system from the GNU project. GRUB allows the user to have several operating systems installed and when you turn on the computer, select one of them to boot. - Taken from Wikipedia.

NTLDR (Reduction from the English. NT Loader) -Rearter Windows NT operating systems. NTLDR can be running from both a portable device, such as, for example, a USB CD-ROM or a USB flash drive) and a hard disk. NTLDR is also able to load operating systems that are not based on NT technology, by referring to the appropriate sector in a separate file. [1] To start NTLDR required, at a minimum, the presence of the following two files on the active section: NTLDR, which actually contains the loader code , and boot.ini, in which commands are recorded to form a system selection menu and parameters for its startup. To upload operating systems based on Windows NT, you also need to have a Ntdetect.com file. - Taken from Wikipedia

Chameleon cross platform bootloader.

Bootmgr bootloader used from Win 7 to Win 10.

07.02.2018
still interested in multi-loading, here are a few examples of it, there are a lot of roughness in the text, if you point me to them, then I will try to fix it.

To everyone who is mastering the OS multi-boot or is interested in it, I want to offer the option of installing and sharing on one PC, two common OC. This is Windows 7 and Ubuntu 17.10
Although Windows 7 is losing its relevance, it still has a lot of admirers and, in addition, the simple and convenient NTLDR bootloader with the BOOT.INI text configuration file

But as you know, the best enemy of the good, then in the second part of this material we will consider the same thing instead of Windows 7, but with Windows 10.
It is also important that we consider the option of multi-boot with the old classic BIOS, so if you have a computer with a new UEFI, then later we will consider your option too.

There are many articles on how to reinstall Ubuntu to an existing one on the Windows disk, it is quite simple to do. Although there are several pitfalls and potential dangers. The first danger is the re-allocation of the HDD, the second danger is an incorrect modification of the primary bootloader. errors in re-partitioning are fraught with data loss, errors when changing the bootloader will lead to the inability to load the previous OC.
How to protect yourself and reduce the risk? this can be done by learning how to restore the original bootloader and reserve your data. these are your photos, videos, texts, etc.

ideally, they will generally learn and do all this not on the main PC, but on the secondary one. unless of course there is one.

Ubuntu developers are realistic and designed their installer in such a way that when installing on a hard drive with Windows already installed, their system can automatically re-partition the HDD and replace the MBR bootloader. and at the same time not to disrupt the operation of the previous system, while maintaining the ability to load it. it is very convenient and for most users this is enough.
Here are the links to these materials:
https://losst.ru/ustan...ntu-ryadom-s-windows-7


but if you want to understand and master multiboot in more detail, you will have to make a little more effort. but as a result, you can install any number of any OS on one HDD and in any sequence.

There isWindows 10, Android, Phoenix OS.
I do not need Phoenix OS.
Phoenix on grub

https://www.acer.com/ac/ru/RU/content/home
Acer Aspire Z1-622
UEFI

Moreover,nowherethe boot disk is not displayed.
Even in Acronis (bootable flash drive).
And, everything is installed on 1 disk.