So, basically what I want to know is, can I move my steam games, and other games like overwatch, from my current windows 7 machine, to my new windows 10 machine? Or will I have to download them again? I will be moving them using a secondary HDD.
The simplest way to do this in Steam is by using Steam's built-in backup & restore feature. You can back up your Steam games either individually, in groups or all at once to a folder on the secondary HDD then restore them after installing Steam on Win10.
For Battle.net (i.e. Overwatch), you need to get a bit more "hands-on". This post on the Battle.net forum outlines how to move games from one computer to another and would also apply to moving games from an old Windows install to a new one.
Similar tricks may work with other platforms. Google is your friend here - Googling for "origin backup games" turns up an article on EA's help site that outlines how to back up and restore Origin games - basically the same procedure as for Battle.net.