How does react understand when to contact the server and when to render a component?

Question

React

How does react understand when to contact the server and when to render a component? Let's say at site.com we get a react app that uses react router dom, we have a router that, if it doesn't find a suitable route, displays page not found. what if we have data from the server pulled from site.com/api? react will give not found page or load data?