When you start up Windows 10 for the first time, Windows Defender is on and working to protect your PC by scanning for malicious software. It will turn itself off if you install another antivirus app. Windows Defender uses real-time protection to scan everything you download or run on your PC.
If you need to temporarily turn off Windows Defender, select the Start button, select Settings > Update & security > Windows Defender, and then turn off Real-time protection. Note that while real-time protection, your PC may be vulnerable to threats.