Nothing to do with React, it's a common optimization to improve performance with long lists. You only render the dom elements in the viewport, with some buffer. A common technique to achieve that is called "virtualization" or "windowing".

It's common enough that there were a couple browser proposals to deal with this and would address the Ctrl+F issue. I believe this has been merged into the CSS Containment spec, but at the moment it doesn't make windowing obsolete in every situation.

It's not even a technique limited to browsers. When I did Android development years ago it was a technique used in native list controls to reduce the overhead of them. Though in that case, there's no search based on what's been rendered like a web browser has. And of course if you did implement search, you could have that look at the underlying data, so it doesn't matter if it's been rendered.

But Web browsers already don't render off screen content right? I'm pretty sure I remember opening hundreds of megs of data in Firefox at one point without an issue. Or old reddit with RES has infinite scroll and you can go dozens of pages deep without a hitch. All while not lazily rendering the other pages.