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.