After doing research on Google, I couldn't find answer to this question. Is Hard Disk Drive a part of motherboard or not? When talking about Tower Case System Unit, the HDD is seperate from the motherboard but connected to it. So, does it mean that HDD is the part of motherboard?
A hard-drive is never part of a motherboard.
However, there are SSD hard-drives that connect directly to the motherboard and work way faster than SATA cables.
Check out NVMe PCIe M.2.
I have one, and Windows 10 loads in less than 2 seconds since power on. It's also due to fast memory and processor, but it's 100% contributed by this hard drive too.