Why "in" and "here" is used together? How are "What's going on in here" and "What's going on here" different?
This is a good question. In this context, both "what's going on in here" and "what's going on here" can be used to express the same general meaning. "In here" is often used to refer to a specific place, usually one that is enclosed in some way (like a room). Thus, we can interpret the speaker as meaning "what's going on in this room" with the "in here" meaning something like "in this room." Generally, you have a choice to use either, with "in here" meaning "in this specific, enclosed space," while "here" means "this location." Ex: A: Where are you? B: I'm here at work. (This location) Ex: There are so many mosquitoes in here! (This room/enclosed space) Sometimes, both can be used, like the example in this video. Ex: It's crowded in here. (Enclosed space) Ex: It's crowded here. (General place)