Snap layouts help arrange open windows so you can read a source while writing notes or compare two documents.
Try a two-window layout
Open the first window and press Windows key + Left Arrow. Select a second open window for the remaining space. You can also hover over a window’s Maximise button and pick a Snap layout.
Adjust your setup
Drag a window to an edge or corner to position it. Layout choices depend on screen size and the windows open. Put reference material on one side and the document you are editing on the other.
If snapping does not work
Check Settings > System > Multitasking and ensure Snap windows is turned on. Some apps may behave differently; test with File Explorer and a browser before troubleshooting one particular app.
Further reading: Microsoft Support: multitasking.