Setting up a passcode also activates data protection, which encrypts the data on your iPhone or iPad so that only someone who knows the code can access them. It seems that no one has responded for a long time. To start the conversation again, simply ask a new question. The code is used to unlock a device's lock screen, while the Apple ID password is used to access your Apple account services and to iCloud.
The password for downloading apps is the password for your Apple ID. If you only have one Apple ID, it's the same password you used to sign in to this forum. In general terms, an access code is a numerical security code: all numbers. A password is an alphanumeric phrase (preferably a meaningless phrase) that uses numbers, letters, and special characters.
If you're wondering if they're the same thing, no. A password in the context of Apple is the string of letters and numbers that you use when you sign in to your Apple account to access an Apple service. It's likely that you used yours to post on this site. The code you use on one device has nothing to do with the code on the second device.
It also doesn't have anything to do with your Apple ID. I have the same passcode for my iPad and my iPhone, can I change one of them?.