How can I add a toggle button to my online store's cart?

[Bligoubloups]

Hello!

I want to make an "app" I guess that adds an element like a toggle button in the cart.
I don't know if I have to make an app or an app extension and if it is the right way to do it.

I don't know either if I should parse the cart html to insert my stuff or if there is an api for that.

Thank you for your help!