How do I maximize security and privacy on my iPhone 11 running iOS?

Probably the weakest aspect is the human factor.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Avoid scams when you use Apple Cash - Apple Support

About Gift Card Scams - Official Apple Support

Identify legitimate emails from the App Store or iTunes Store - Apple Support

Security and your Apple Account - Apple Support

Two-factor authentication for Apple Account - Apple Support - "Two-factor authentication is an extra layer of security for your Apple ID designed to ensure that you're the only person who can access your account, even if someone knows your password."

If you are a very important person who is being chased for your millions of dollars and fame:

About Lockdown Mode - Apple Support

Apple expands commitment to protect users from mercenary spyware - Apple

Account security:

Safety Check for an iPhone with iOS 16 or later - Apple Support

Personal Safety User Guide - Apple Support

You express two conflicting thoughts above:

• Maximum Security - Maximum iPhone security is achieved using Lockdown Mode.
  • Harden your iPhone from a cyberattack with Lockdown Mode - Apple Support
  • From the above article: “Lockdown Mode … should be used only if you believe you might be targeted by a highly sophisticated cyberattack, such as by a private company developing state-sponsored mercenary spyware.”
• Recommendation - The best security safeguard is your diligence. Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support