Flip the Ring/Silent switch on the left side of the iPhone to Silent
Open Control Center and tap the bell icon if available
Go to Settings
Tap Sounds & Haptics
Turn off Change with Buttons if you want volume buttons to stop changing ringer volume
Drag the Ringer and Alerts slider all the way down
Turn on Focus mode from Control Center
Set a Do Not Disturb schedule in Settings
Turn on Silent Mode for specific apps in Settings
Lower media volume with the volume buttons
Turn off vibration in Settings if needed
Enable Sleep Focus at night
Use the Action Button to toggle Silent Mode if configured