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The atmosphere is not stratified into layers of different gasses. All gasses are mixed throughout the atmosphere, so you also find enough Helium at the bottom where we live. Indeed, all gasses in the atmosphere get less dense with increasing height, so helium has the highest pressure/density at the ground. [1]

At the same time, lighter gasses have vertical distributions that reach higher up into space, and some helium does get lost as it "boils off" from earth over time.

[1]: https://chemistry.stackexchange.com/a/61154/14998