Like other antivirus apps, Windows Defender automatically runs in the background, scanning files when they’re downloaded, transferred from external drives, and before you open them.
You don’t really have to think about Windows Defender at all.
Antivirus definition updates automatically arrive through Windows Update and are installed like any other system update.
These types of updates don’t require rebooting your computer.
If you have a huge photo or video library that you know is safe, you don’t really want scanning slowing down your editing.