I don't know how to use an another SSD

So basically, I recently change from Windows to MacOS, and i have a problem.. I am running out of space. And i have a selution: I have an another SSD build in into the IMac, but there is a catch; I don't know how to change the main SSD! I tried googling the problem and there is zero results that suit my needs... so i ask you guys, HOW CAN I DO THAT?!?! (I pinned an image to this post :))