Windows 10 Pro OEM key

Hello,
last month I purchased a Win 10 Pro OEM Key from zero&nine using the Britec discount code and was watching todays Britec video about upgrading Win 10 Home to 10 Pro

I am not planning to use my Win10 Pro OEM key to do this, as I have other plans for it involving a new build, however...

I dont fully grasp the OEM concept, and would like to improve my understanding, can someone help out with these 3 questions please

1..if I tried to use this Win 10 Pro OEM key with my existing Win10 home OEM key what would happen?

2..if it proceeded like the video and changed it to win 10 Pro, have I lost my existing Win 10 Home key for good?

3.. is the validity of the OEM key to do with the drive, BIOS or the motherboard?

thanks
cw2879