How do apps (phone and desktop apps) connect to the internet and access other system files, such as, contacts, storage,etc?

Question

This might be quite obvious but I am quite new to this stuff and would appreciate if someone could explain. If the question is confusing I will provide some examples: Youtube app, for instance, how does it connect to the internet and also how is it able to access your camera if you want to record a video? I see how browsers can do it - They are meant for you to connect you to other websites and be able to view those content, but how do apps do it?
I would really appreciate if you could link me to something or explain it yourself. Thank You!