where I can find my iPhone passcode?

You cannot find your passcode unless you actually wrote it down somewhere for future recall. If you have forgotten your passcode, you'll have to erase your iPhone using a computer, which you can then restore from your most recent backup (assuming you are backing up your iPhone as you should be doing) after the phone has been reset.

This link provides the directions you need --> If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

Apple does not store your passcode......it is up to you to remember it for security reasons.

Did you forget your passcode?

If you forgot your iPhone passcode - Apple Support

It might be a good idea to write down your passcode and store it in a safe place......even if you are sure that you will never forget it.

I don’t know specifically what context your need for a “passcode” is, but often it can be either your Lock Screen passcode or a 6-digit single use passcode which should be sent to one or more of your trusted devices via text message or on-screen pop up.