At both HKG and ICN you can transit/turnaround without clearing immigration.

If it is all on one ticket then easiest is to get the return boarding pass when you check in for the first flight (at SIN or elsewhere). If not then you'll need to visit the transfer desk.

At HKG transfer desk is opposite immigration (well there is another down the pier but this is not near the gates SQ uses). Go here first before security as security won't let you through without boarding pass (itinerary or ticket is not good enough).

At ICN need to show security onward ticket, transfer desk is upstairs on departure level opposite where you come out from the escalator/lift from transit security.

Such transits are very easy at both airports.