It depends massively on the airport layout. If you transit domestic to international at Heathrow Terminal 5 you don’t need to re pass security, just a boarding

It depends massively on the airport layout. If you transit domestic to international at Heathrow Terminal 5 you don’t need to re pass security, just a boarding pass check and can stay in the sterile zone. If you fly out of another terminal though you have to transfer to the airside non sterile zone. If you transit at Helsinki from the U.K. to the far east then no security check needed, but if you transit from Helsinki through london to Africa you do go through security Some airports just do security at the gate. Some airports have multiple security at the gate - the airport/country, then a second security check by the airline. Then there’s the spot checks, I’ve been through a second check occasionally flying out of the U.K. (MAN and LHR) to the US, normally don’t get it, but sometimes do. There are no hard and fast rules.