Passcode!

If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -

Click on the link for instructions on how to reset your passcode without a computer if you have iOS 15.2 or higher.

How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

Click on the link for instructions on how to reset your passcode using a computer.

If you forgot your iPhone passcode - Apple Support

If you did not write it down or you cannot recall the passcode, your only option is to follow the instructions in the Apple Support document above your name, or this one below.....if you don't want to use a computer:

How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

You don't.

The pass code is stored on the device and never leaves the device. No one knows it but you.

Follow the instructions in the linked article.