How does plasma membrane permits interactions with the outside enviroment?

It is permeable to certain substances. Gases diffuse [simple] across the plasma membrane, small uncharged polar molecules [ex. H2O, ethanol], and steroids diffuse also. Other molecules diffuse based on the size and amount on both sides of the membrane [gradient].

There are also special membrane proteins that facilitate the movement of molecules from the outside to the inside.

And there are pumps that act to move molecules or ions against the concentration gradient with help from energy [ATP].

Hope this answers your question.

proteins, passive and active diffusion.