The aliens portrayed in motion pictures are entirely imaginative. Since we have not come across any alien species yet, we cannot know for sure what they look or whether they actually exists.

There is a famous proposition on this, called The Fermi Paradox
https://en.wikipedia.org/wiki/Fermi_paradox

Also you're assuming aliens need oxygen to survive or feed on plants. This is necessarily not true. All life on earth is carbon based, aliens don't have to be so. Feeding, mating and passing down genes are also our traits, they don't have to be in every case.