Source of active mark in Windows 7 System Reserve partition

My flash drive is MBR partitioned( has active mark)with FAT 32 file system.When I install Windows 10 in UEFI mode via a Rufus bootable Usb flash drive with GPT partition & FAT 32 file system,the active mark disappears.After further research,found out that partition can be marked as active only on MBR disk & the active partition like in Windows 7 System Reserved houses the boot files. My question is how does MBR disk gets partition(System Reserved in Windows 7))marked as active.What is the source?Is it in the Windows OS or is done during installation process or is it included in the Master Boot Record files?

Being "active" is a flag set in the MBR partition table in order to allow the firmware (BIOS) bootloader to be able to detect which partition should be used when booting.

The BIOS firmware reads the partition table, checks for an "active" flag, and then loads the bootsector from the partition that is listed. That bootsector then loads the OS bootloader and booting the OS continues.

GPT does not use an "active" flag. The UEFI firmware looks for a FAT32 partition, then scans it for UEFI executables that can be used to load an operating system..