Question! So I bought a used windows laptop. Changed out the hdd for a ssd but how do I find out what the serial is for the one on the hdd? The sticker fell off the laptop before I got it and for some reason Windows 10 will only display the last 4 digits.

It is saved on the motherboard. You can just install windows 10 and it will find it itself.

Its not saved onto anything, It registered to the motherboards hardware ID#.
When you reinstall it sees the same hardware ID and activates it.

Actually, prebuilt units that come with Windows 10 have the product key embedded in firmware. So those are saved on the motherboard. DIY builds, upgrades from Win7, etc are not, ofc.

try lansweeper

If it's a pre-built it will be in the mobo firmware. otherwise there are programs that can read all your product keys; magic jelly bean has a decent free one iirc.

download lansweeper and run it once.

its a freeware and easy to use