This may help. It's a guide to the equation describing lift as a function of blade angle of attack, air density, and true airspeed.

https://sciencing.com/calculate-lift-rotor-blades-7680704.ht...

Airplane/helicopter mass * g * time

Assuming the plane/helicopter didn't accelerate up or down and assuming there's no wind.

Sure, that has to be true if the helicopter can hover. But how do you know that's possible?

You can't postulate that something flies as part of an explanation why it can fly...

You can solve for the lift generated by each rotating blade. Then it's roughly the same calculating the lift for a fixed wing, except for the freestream flow speed varying along the span. The lift is different at each spanwise location, so you integrate to get the total lift for one blade, then multiply by the number of blades. Numerous ways to do this, but the simplest accurate way is via lifting-line theory. The resulting system of equations is solved iteratively.

Why not? I can see it is flying, no point in questioning that.

There are some invariants to the world, I can infer information from those invariants.

Just like if I weigh a bottle in the morning, and it shows 3kg, the. I weigh it in the evening, it shows 2kg - I'll know that net 1kg has left the bottle. Doesn't mean I know how.