It’s for a fan.
That fan can be powered either with the Molex connector (big chunky loose/floating bit) or a PWM fan connector (smaller one plugged in to the motherboard).
You wouldn’t plug in both of them at the same time.
Since there’s already a connection directly to the mainboard, the bigger connector was already disconnected and probably tucked under the IO shield.l that you taped it to.
If you want, you could also route the cable over to the back of the chassis and plug it in to the power supply’s Molex cable. Instead of having its speed regulated by the motherboard, it would just run at 100% all the time.
The upgrade is not automatic. You interacted with a prompt to approve the upgrade, you just might not realize it because it may have been on impulse.
I manage Windows installations for tons of folks and I’ve never seen the level of repeated prompting / nagging you’re describing.
For anyone who has wanted to stick with 10, it has been enough to decline the upgrade from the full screen prompt and then choose “Stay on Windows 10 for now” from Windows Update.
It’s possible that your registry changes had something to do with your unusual experience. I run into a lot of folks who complain about OneDrive “ruining their computer” after they’ve tried some obscure method of disabling it when all they had to do was uninstall the program like any other.
Don’t get me wrong, there’s plenty Microsoft does wrong but compared to the Windows 10 shitshow I’ve found they’re actually handling this transition quite well.
Reminder that a lot of these design trends are intended for the average basic user, not power users with strong preferences. They make up the majority and need quite a bit of handholding.