BOOTMGR eventually executes winload.exe, the system loader used to continue the Windows boot process.
Configuration data required for BOOTMGR can be found in the Boot Configuration Data (BCD) store, a registry-like database that replaced the boot.ini file used in older versions of Windows like Windows XP.
The BOOTMGR file itself is both read-only and hidden and is located in the root directory of the partition marked as Active in Disk Management. On most Windows computers, this partition is labeled as System Reserved and does not have a drive letter. If you don't have a System Reserved partition, BOOTMGR is probably located on your primary drive, usually C:.
BOOTMGR, together with winload.exe, replaces the functions performed by NTLDR in older versions of Windows, like Windows XP.
A common startup error in Windows is the BOOTMGR is Missing error.