How to safely transfer program files from SSD to 1TB HD

So im a newbie to tech, but built my first PC about a year ago, and so far so good. the design was supposed to be my OS installed on a SSD for fast start ups, then have a 1TB HD for all my games, programs, etc. Well i started noticing some performance issues then a noticed popped up “full storage” turns out everything from day one has gone on the SSD which is now full. I don’t know how to transfer everything except OS to the larger HD and know its possible to screw up program files. is it simply drag and drop in file explorer? So many files that i dont know what they do.