I have read in an authentic Islamic Psychology course that the harm by an evil eye is not the eye itself but it’s by an evil jinn.

Who are the evil jinn?

What if a person has high imaan and does righteous deeds like praying, doing zikr, etc; will it be difficult for the evil eye to have an effect on him/her?

Yes, a person who has high Iman and protects himself from the evil of jinn and evil eye, then Allah will ward off such evil from him. As for doing righteous deeds like praying and saying one’s daily zikr, this protects from jinn and evil eye in proportion to how effective those deeds are, which is based on sincerity, certainty, understanding the words one says, etc.

And Allah knows best.