Can my iPhone 6plus update to iOS 13 ??

Sorry, no. 12.5.5 is the highest version.

That's not something that I know anything about. There are two (or more) PS4 apps on the App Store that work with 12.5.5 and one (or more) that don't. If the app that you need doesn't work with iOS 12.5.5 I don't know of anything else you can do.

Someone else here with more knowledge of PS4 may reply with more information.