This is for learning purposes only. A real Windows bootloader (like bootmgr ) is proprietary, digitally signed, and interacts with the Windows Boot Configuration Database (BCD). You cannot replace it with a custom bootloader on a standard Secure Boot-enabled PC without disabling security features.
The Windows Boot Manager is notoriously aggressive when it comes to dual-booting. bootloader for windows 10
The system firmware (BIOS or UEFI) performs a Power-On Self-Test (POST) and locates the bootable device. This is for learning purposes only