You can find thousands of discussions about this so I'll keep it simple. It is NOT as if your phone can use that as part of the internal space. It could be used for photos, videos and sometimes for apps. It's not seamless and for the reason that it's all over the web, I will keep this short.
I have to guess you are talking about an Android phone. You did well to skip those phones under 8GB. My bare minimum for Android is 1GB RAM and 8GB internal storage. But with 88 buck phones having 2GB RAM and 16GB internal storage I can't see why we would go with less than that (see the Bluboo Picasso on Amazon.)
I'm one of sixteen people left in this world who doesn't own a smartphone......yet! I'm about to take the plunge and would appreciate a little help in learning about smartphone memory.
Essentially, I'd like to know if there's any difference in internal memory vs. added card memory. In other words, and as a "for instance", is there any difference in performance between a phone that comes with 16GB internal memory and a phone that comes with an 8GB internal memory and has an additional 8GB memory card installed for a total of 16GB?