How flying at high altitudes reduces drag and save fuel?

The air is thinner at high altitude. There is less air per volume so there are fewer molecules to slow down your airplane.

As a result you can fly faster with less fuel required to blow through the molecules of air.

The air is thinner.

The Air is less dense. A good analogy is say your walking in water in a pool (more dense than air). It takes a lot more effort. Than when you jump out of the pool, and your surrounds is air. Its a lot easier

the answer lies in your question so 10pts. for your Q & A

The higher you fly, the thinner the air through which you fly, so the lower the drag. If you leave the atmosphere entirely, the drag drops to zero, in which case you can cruise along indefinitely without using any fuel—you’re in orbit.