How do I lock iPhone immediately?

Hold both the side button and volume up button for about two seconds like you are going to power the phone off. At that point the iPhone is instantly in the Before First Unlock (BFU) state which disables even Face ID until the passcode is entered.

There is another setting...

Settings, FaceID& Passcode, Require Passcode, Immediately

Ah! Good catch, LD150. I've never changed the setting from the default, and didn't even know it was there. If put it on any setting other than Immediately, swiping up puts you right back at the desktop without asking for your passcode.

Don't know who would ever want to do that since any crook who steals your phone would have instant access to everything, assuming the timer hasn't yet expired.

Kurt Lang wrote:

Ah! Good catch, LD150. I've never changed the setting from the default, and didn't even know it was there. If put it on any setting other than Immediately, swiping up puts you right back at the desktop without asking for your passcode.

Don't know who would ever want to do that since any crook who steals your phone would have instant access to everything, assuming the timer hasn't yet expired.

Certainly with many phones, if you have Apple Pay set up, "Immediately" is the only option available.

Definitely SE phones, and possibly now all phones (subject of another long thread here).

Kurt Lang wrote:

...

Don't know who would ever want to do that since any crook who steals your phone would have instant access to everything, assuming the timer hasn't yet expired.

I have my ipad set to 1 minute because it doesn't go anywhere, and I am constantly pressing the power button when mishandling or mis-timing the screen shot sequence (I try to press the volume button a fraction of a second before the power button but sometimes get it wrong.)

The 5 minutes timer is immediately cancelled when you press the side button, which turns the screen off. Pressing it again requires the user to enter their 6 digit password.