Go to shimano's support website and download the DM (dealers manual) for your particular model of 105 derailer. That will tell you what does what.

I'd just expect cable tension is the problem if it did gradually over time. If you've laid it over several times and/or banged up the DR, then the hanger might be slightly bent. Most I just replace over trying to bend it back. It's a cheap part usually.

The worst thing is to just start adjusting stuff if you don't know what it's going to do. If you've never worked on one, then realize that the learning curve can be expensive. So you might spend much more to gain experience that the bikes shop will charge to fix it.