Either 2a or 2b can be correct, depending on your meaning.
The other day, someone had turned the thermostat up, and it got quite hot in the room I was working in. I said "It's really hot in here!" In this case, I was referring to the room.
A couple weeks ago, I went to Texas to visit my parents. When I left home, it was cold enough to wear a coat, but when I arrived in Texas, it was so hot I wore shorts and never touched by coat. I said to my husband, "It so hot here!" In this case, I was referring to the overall geographic area.
More people fiddle with the thermostat than travel hundreds of miles to a different climate in any given week, so "It's hot in here" is more commonly said, but no more correct.