The Settings

You'll be needing to capture as much light as you can from some very far away objects... So let me explain what settings are best for you!

ISO (2000 - 6400)

You need to set your ISO as high as it can go. ISO makes the camera sensor more sensitive to light (at the cost of introducing digital noise to the photo, but this can be easily removed in Photoshop). Your phone will have an ISO limit, but my personal recommendation is 3200 ISO and to not go above 6400 ISO, otherwise there will be too much digital noise.

Shutter Speed (20 - 30 seconds)

30 seconds should do it. I wouldn't go any longer than 30 seconds otherwise you will start to see the stars trailing in your photo. At 30 seconds there will be some trailing, but not enough to notice on a phone screen or computer.

Focus (Set to'Infinity')

You are trying to focus on objects that are so far away that its difficult to comprehend.

So you just need to set you focus to infinity (AKA as far as you camera can focus). For the OnePlus 5 this focus is denoted by a small mountain symbol.

Aperture

The only phone that can adjust aperture is the Samsung Galaxy S9. In this case, you want to use F/1.5 and not F/2.4. This is because the lower the 'F' number, the wider the aperture blades become! And you want that as wide as you can to let in as much light as you can (My OnePlus 5 has an aperture of F/1.7).

Taking the Photo

Unfortunately not everyone can just go out to their backyard and point their camera at the sky to capture the Milky Way. You need to be away from light pollution, which for me is about a 40 minute drive north away from the city into the country.

To check where light pollution is low where you live, use this website: https://www.lightpollutionmap.info/

For reference, I took my photo somewhere around that red circle.