When you switch over to Modeling Mode (Shift+5) and go to Create Tab, you'll have two Output Types you can work with in UE5: Static or Dynamic.
Static Meshes are saved inside the Content Browser. You can re-use these Static Meshes in any level you are working on. Static Meshes are what you'll be using to create most of your world geometry.
Dynamic Meshes are only contained and saved within the level itself and NOT saved inside the Content Browser. With Dynamic Meshes you are free to focus on environment blockouts without having to worry about hundreds of mesh iterations cluttering the Content Browser. However note that Dynamic Meshes are not efficient for rendering and memory and are not compatible with Lumen or Nanite. This is why you should only use them as blockouts and not as final world geometry.