Your Apple ID is going to do the job just fine. You do not need a developer account to download the app. You can download the latest, stable version of Xcode from the Mac App Store. There are quite a few apps that you can use to edit a Plist file on macOS but the best, free option that you have is Apple’s own Xcode app.
To that end, you can no longer open these apps with TextEdit, and you need a special app for it.
That’s changed now since these files are rather important and Apple doesn’t want it to be too easy to change them, especially if the edit is being made by a malicious app. A Plist file used to be really easy to edit on macOS and the stock TextEdit app was enough to do that job. The contents of each file are different of course but you’d be hard-pressed to find an app that doesn’t come with one. Plist files are an essential file that all macOS apps have.
