Answer 1

I am assuming you are refering to sibling components OR accessing a state of a child from it's parent (those are the tricky ones)

One way to go about it is having the state defined on a top level and passed along as props along with a function to update it.

That's a rather straightforward solution.

A better way that will require some refactoring (and some extra learning) is to add a global state management to the project, such as Redux, Mobx or any other (there are plenty out there)

Redux is one of the most popular today, and Mobx is a personal favorite of mine.