What is the difference between HHC and a regular line company? Specifically combat arms.

The HHC has the enablers and the staff. People often say that a HHC is a harder company for the Commander and First Sergeant to lead. Nope. Is it more challenging to get and keep everybody up to standard on 350-1 training, weapons qual etc? Yes, because unlike a traditional "line" company if you decide to do training you could shut down the entire staff. That will get the BN CDRs attention. I see from your profile that you are in the maintenance field. Unless you have been assigned a position on the staff as a driver, I suspect to you the change will be unnoticeable.

HHC = Headquarters & Headquarters Company. It is the command and control element of a BN or BDE whose mTOE contains the Commander, CSM and principle staff (S1-S6).

HHC will have all of your staff sections S1, S2, S3, etc. It will also have the heavy 120mm mortars, scouts, and snipers. As a young Infantryman, chances are you will either go to the scout Platoon or S3 (Operations) as that's where the 11B slots are.