Question

Does windows security need to run in background

Answer 1

Windows Security always runs in the background unless you force stop it.
It boots up when you turn your pc on just like any other antivirus system.

Answer 2

Windows Defender automatically runs in the background, scanning files when they are accessed and before user open them. When a malware is detected, Windows Defender inform you. It won't ask you what you want to do with the malicious software it finds.