Your iPhone potentially holds your email, your text messages, your password manager, your banking apps, your social media, your photos and your 2FA methods. It deserves particularly good protection.
11.1 Use Face ID
Turn on Face ID and use a device passcode that is hard to guess. Avoid the obvious ones:
000000
123456
111111
date of birth
11.2 Hide your passcode in public
Picture it: someone watches you type your passcode, then steals your iPhone. Apple built Stolen Device Protection precisely to limit the consequences of that scenario.
11.3 Turn on Stolen Device Protection
Settings
→ Face ID & Passcode
→ Stolen Device Protection
Apple explains that this feature adds extra protections for certain sensitive operations when an iPhone is away from familiar locations. Apple also lets you choose "Require Security Delay: Always", which applies the extra measures even in a familiar place.
11.4 Keep iOS and macOS up to date
Updates regularly fix security vulnerabilities.
iPhone
Settings → General → Software Update
Mac
System Settings → General → Software Update
Turn on automatic updates if that suits you.