Does a React.js app get entirely shipped to the browser on request?

This is going to sound like a really dumb question but does the entire React app get shipped to the browser on first page load? Or is it shipped to the browser as-needed? If it is shipped as-needed then who decides what is needed when?

Let's say its a 3-pager. (e.g. /login, /users, /users/:userId).

I can probably just inspect the network calls or curl my server... but I consider that anecdotal and would like to get a more comprehensive idea.