Do both top and bottom attachments have some pushing away system? Or do separation at 61km above sea level allow the use of aerodynamic lift-away force, meaning bottom attachment may be one simpler device? (not pushing away, only unlocking)?
Three things: Unlock, push, and aerodynamics work together to achieve separation.
Erik Seedhouse's book: "SpaceX's Dragon: America's Next Generation Spacecraft" and Spaceflight101's article: "Falcon Heavy Launch Vehicle Overview" explains:
The Falcon Heavy's Core stage uses "Pneumatically actuated mechanical collets" and the boosters use "thrust struts, center pushers, and RCS".
"Stage separation is accomplished via separation collets and pneumatic pushers in four interfaces connecting the two stages. SpaceX tries to avoid using pyrotechnics for separation events. A fourth pusher interface was added to make the separation between stages more reliable.".
This video: "Falcon Heavy Test Flight" shows the pneumatic pusher and struts in operation, perhaps it's easier to see in slow motion (YouTube has slow motion in the Settings).