Do you have to go through security even it’s a connection for BOS-FRA

Yes, need to go through security.

Yes, one has to undergo security check for all flights however for domestic destinations u are exempt from immigration......happy flying

Not 100% sure, but I think you might depending on where you are arriving from, and its an intl flight. Check what terminals you are arriving to and departing from. Generally intl flights are Terminal E

Sure.