How does a fixed-wing, firefighting aircraft,manage to fill its scoop and remain airborne?

Answer 1

Good question!
"Catalinas" were the first airplaine to do that, and are now manufactured by Bombardier (Canada and France) and called "Canadair".
It is a high wing twin engine with a load capacity of... 6700 litres of water. That's 6,7 TONS on water, loaded (or scooped) in less than 3 minutes.
So the weight of the aircraft, skimming of the water surface at "near 0 feet", increases by 6.7 tons: NO pilot can control that change of weight. The aircraft WILL get into a nose dive... and crash, even if the pilot pulls hard on the stick!
So there is an autopilot.
The aircraft levels above the water.
The radar measure the distance of the water,
The autopilot lowers the scoop and the aircraft,
Water rushes in,
The autopilot "pulls" the stick to keep the aircraft level exactly where it should be,
When full, the autopilot pulls the stick up just a little bit more and the scoop is withdrawed.
The pilot can now take over.
At all times when the AP is engaged, the human pilot has control on the aircraft on the rudder (to turn), and on the elevator (only to pull up). The ailerons/rudder, normally used to make a turn, are under the AP control to maintain level flight.
PS: It CANNOT be sea water in fire fighting: the salt would destroy what remains of the soil and kill everything else...

Answer 2

they swoop down and collect lake or ocean water..