Can living organisms be a habitat? What changes in the internal and external structure of organisms occur depending

1) Can living organisms be a habitat? 2) What changes in the internal and external structure of organisms occur depending on the environment? 3) What is the role of parasites in nature?

1) They can. And they are. For example, for bacteria
2) Living organisms have features characteristic of their habitat that contribute to adaptation (color, physiological features). For example, ostriches have strong legs and can go without food for a long time.
3) Parasites help prevent any creatures from actively reproducing. They are also used in medicine (leeches) and agriculture.