GRUB is a boot loader which is 100% independent of any and all OSes. Grub is installed and run at the boot sector. At the hardware level a system will POST and once it passes POST it will load the boot sector of the HD to then start the process of booting, which is called a boot strap.
During the installation of Linux, if you have a current windows installation it should detect it and automatically configure the boot loader to have options to boot from all OSes installed. You can literally run dozens of different OSes on your computer and grub can manage them.
That being said, when you install Linux it should list all partitions and OSes installed, it will ask you to resize, and it will automatically load that information into GRUB, LILO, or whatever boot loader you use. With GRUB being the most popular.
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