So, if the company doesn't reply or mail you back after a video interview, do you mail back or simply assume you are rejected?

Until you get an actual offer, you always assume you're rejected.

Thank them for the interview via email within a couple hours of its end, and if you don't hear back after two or three days, call them up to check in. Don't email the check-in. Call. If they're considering hiring you, they will say so. If they give you a noncommittal answer, move on.

If they are interested, they will get back to you. If they don't, they aren't, and calling them is just a waste of time.