I’m gonna upgrade a bunch of parts in my PC such as motherboard, RAM etc. However I will not be changing HDD or SSD’s which currently contain my Windows on etc. If I changed any of these will I lose my files or have to reinstall Windows?
You will have to reinstall Windows if you change the motherboard.
You won't lose any of your data if you back it up to another storage device (which you should have done anyway even if you're not changing anything as all hard drives eventually fail one day---- so without backups you lose everything when that day comes.