How can I download apps without having to sign into iTunes

To update apps you don't sign into iTunes, you sign into the App Store. In fact you don't really sign into either:

"When you sign in with your Apple ID on your iPhone, you will be signed in to the iTunes Store, App Store, and Apple Music automatically so that you can make purchases or begin using Apple Music quickly and easily."

From: Apple ID & Privacy - https://support.apple.com/HT208650

Wrong. If you fully restarted your device & sign into your Apple ID, iTunes won’t automatically sign you in.