What happens when a iphone gets turned off?

Does everything such as apps, pictures, music all go way? Should I do a factory reset I'm supposed to be getting a new plan so I just wanted to know.

If you're talking about a data-plan or carrier service provider, no, all your data is safe.

I think you meant deactivated & nothing happens to the content. That just means the phone can't be used to make calls/texts/data

Nothing happens to your personal data on the iPhone. All changes happen only on the sim. If you change plans and not providers then you don't even have to change the sim. New plan is automatically activated on the date your new plan starts.