Force restart (iPhone 8 and later): Press and release Volume Up, press and release Volume Down, then press and hold Side button until Apple logo appears
Force restart (iPhone 7 and 7 Plus): Press and hold Side and Volume Down until Apple logo appears
Force restart (iPhone 6s and earlier): Press and hold Home and Side (or Top) until Apple logo appears
Reset all settings: Settings → General → Transfer or Reset iPhone → Reset → Reset All Settings
Erase all content and settings: Settings → General → Transfer or Reset iPhone → Reset → Erase All Content and Settings
Erase iPhone using a computer (Finder/iTunes): Connect iPhone → select device → Restore iPhone → confirm
Reset while keeping data (if available): Settings → General → Transfer or Reset iPhone → Reset → Reset Network Settings (or Reset Keyboard Dictionary / Reset Home Screen Layout)
Sign out (recommended before erase): Settings → [your name] → Sign Out → turn off Find My iPhone if prompted