It depends on the state. In many states, regardless of whether you’ve registered in person or remotely, mobile sports bets can be placed from anywhere: your living room, at Buffalo Wild Wings, the bathroom, etc.
Some states only allow mobile sports bets to be placed on casino premises (or another licensed gaming facility like a tavern). The state is able to enforce this policy through geofencing technology. Basically, the location function on your phone has to be turned on to place a wager, and the app detects where you are located. You must also be located within the state where you are placing the bet. In other words, you can’t book a bet at an NJ online sportsbook if you’re hanging out in New York.