If you are using the stock iPhone headphones, they have the button embedded on the cable (see below). To invoke the “next” command tap on that button twice quickly.
If you are using AirPods Pro, you can invoke the “next” command by quickly pressing the force sensor (see below) twice.
If you are using AirPods you can double tap them to invoke some command. By default it is Siri, but you can use the system Settings app to customize this command to “next”.
If you are using CarPlay you should be able to see the next button on your vehicle screen on Now Playing and Dashboard pages.
If you are connected to the Bluetooth hands-free in your vehicle you should be able to find the next button on your steering wheel or near the vehicle screen.
In other cases (like headphones made by some other manufacturer) please consult your manual on whether the next button is available and how to use it.