What needs to be included in app credits?

In all apps there is a "help" or "about" button which displays some info like: author, publisher, designer, graphics, music, licenses, copyright, year, email, app version, maybe some terms and conditions etc. etc.

What info do I must/can/should include in credits? Do I have to include software I used for making music/graphics?
Sometimes there is (C) copyright 2020 [author] blah blah. How to put credits properly and legally correct?

The  (C) copyright 2020 [author] needs to include the date for it to be legal.

As what what you HAVE to display, it's mostly up to you.

If you're using assets which aren't yours and use of them stipulates that you must give credit, then you must mention it.

Same with software. If a condition of use is that you credit it, then again you must, but most software won't ask you to do this. Some free software might, I can't imagine any paid software would.

App version might be useful if a player runs into issues they can let you know which version they're running.

It's worth pointing out that the rules regarding copyright may depend on where you are. In the US, for instance, copyright is established the moment your work is created: https://www.copyright.gov/help/faq/faq-general.html#mywork

You should check with your country's copyright office whether or not you need to register your work, use a particular format, etc.

I'd be interested in knowing what is required or recommended as far as privacy statements.

A link to your support page would probably be useful, too.