Another very important thing to know is that none of us can share the same physical atmosphere because our bodies have developed and evolved along with our own worlds.
No alien race can survive here or on each other's worlds.
They don't adapt.
They can't be openly exposed to our elements.
They visit by natural abilities and by technology.
They use dimensional technologies to effect physical contact with us and each other world to world.
The materials that make up our bodies are the same materials that are natural to our own worlds.
We cannot breathe each other's air or be exposed to each other's elements or chemicals.
Most cannot speak each other's language sounds either.
However, there are many ways, both natural and technological, for advanced races to visit and communicate with each other and with us in person.
These are the things that make the advanced races so incredibly mysterious to us humans, but they are normal and natural.
Some day we too will be considered advanced and be traveling to our neighboring worlds to visit.
Zylem can't live on earth. but his race is the longest to survive here if they got stuck.
They could live up to two hours, but unconscious and dying.
No alien people are living on Earth or in Earth at all.
Alien people may only visit each other's worlds, and only for a short time and in strictly controlled manners.
Think of this as the same as when humans "travel" to the bottom of the ocean, only think of the water as made of fire or acid.
This is why most immediate contact of people world to world is done by spirit - out of body and by telepathy.