Most of the youtube tutorials for air layering I’ve seen completely seal the air layer environment. At most, they put a couple of tiny knife-pokes in the bottom of the seal to ensure water can’t penetrate and saturate/drown the medium. A lot of the tutorials are actually farmers or nursery operators, so their livelihood actually depends on the success of these techniques. I’d recommend searching “air layer” on youtube and just watching as many videos as you can.
Occasionally, I’ve seen a dry-ish medium when they open the air layer, but that’s because they’ve grown tons of roots that have drunk most of the water in the sealed environment. But the roots are still happy and healthy.
The canopy is still getting the vast majority of its water from the existing root base in the ground. The water in the air layer medium is only being taken up to sustain the tiny roots the tree grows into that medium. If there ceases to be water there, I think the tree would just pause growing those new roots; I think it would take an extremely long time for an initially moist, sealed air layer environment to become completely dry, because it’s attached/adjacent to the open xylem of a living tree, which is constantly transporting moisture to the canopy and, thus, is humid.
Again, I’d just recommend watching as many videos on the subject as possible. After a certain point, I feel like the elements of the technique become sort of intuitive. I only began practicing bonsai earlier this year. I initiated my first air layer on a ficus a few weeks ago, and am already seeing new roots in the clear, sealed plastic air layer environment.