How do I find my passcode?

That would be your AppleID password when using the App Store to update Apps. If it is not in iCloud Keychain, then you’re going to have to work with Apple’s system to reset your AppleID password. See -> If you forgot your Apple ID password - Apple Support

When prompted for a passcode for iOS updates, you enter your screen lock passcode. But if you’ve forgotten that, the one, and the only way to reset it is to erase the device with iTunes. As long as you have an iCloud or iTunes backup, you can restore again from that once you’ve erased the device to reset the screen lock passcode.

But if you forget the screen lock passcode and do not have an up to date backup in iCloud or iTunes, then yes, you will lose data. There Is simply, by design, no way to reset the device screen lock passcode without erasing it.

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

The screen lock passcode? If forgotten, there is nowhere to find it. You can reset it by erasing your device with iTunes (instructions in that blue link in the first) reply.

If you mean your AppleID password, if you use iCloud Keychain, look for it there.