Setting the appropriate aspect ratio on your camera.

Rather than just point and shoot it’s advisable to set the aspect ratio on your camera prior to ‘clicking’.

The aspect ratio is the ratio of the width to height of the photo.
Most phone cameras have 5 aspect ratios viz 1:1, 3:4, 9:16, full and widescreen.
I'll only deal with the first three as they are the most common. 
When you turn your phone to horizontal these become 1:1, 4:3, 16:9.

So which ratio do you use? That depends on two factors: the subject of your photo and what you’re most likely to use the photo for.

If you are photographing a small number of people and wish to include some of the background, then use 3:4, 4:3 or 1:1. If you are photographing a tall building, a statue, or a few people and want a ‘tall and narrow’ image then use 9:16 ie the height of your photo will be nearly twice the width.
Use 16:9 to photograph a landscape. Use 1:1 for a square photo.

As I mentioned, what you plan to use the photo for can also be a factor.
When posting to fb either 3:4, 4:3 or 1:1 is usually the most suitable.
If you plan to insert multiple pics in the one post it’s probably best if they all have the same aspect ratio. 
If it’s a fb story or insta post use 9:16. Any other aspect ratio will result in black bars above and below your image.
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If you think you might want to print the photo in order to display in your home then 1:1 or 4:3 may be the best aspect ratio.

If you think you might run a slide show on your TV the 16:9 would fill the screen.

Similarly you could create a slide show if you have access to Apple IMovie. That also requires your photos to be. 16:9 (if you don’t want the side bars).

There is software similar to IMovie available for Android phones and PCs.

Having said all that, it’s the subject that is the overriding factor when it comes to selecting the aspect ratio.

BTW  a landscape pic when enlarged to a width of 32cm would be 18cm high (16:9).

Of course photos can be cropped to any size. You could, for instance, crop a 16:9 pic to 9:9 (square) by cutting  vertically. 

Many phones have editing software that, among other things, allows you to easily crop your image to another aspect ratio eg. from 16:9 to 4:3.

Check the internet to see how to change the aspect ratio on your phone camera. It takes about 2 seconds. You may want to take 4 photos eg 3:4, turn camera, 4:3, change ratio, 16:9, turn camera 9:16. You’ve taken four photos in  5 seconds max.

In Gallery 2 I’ve have a range of photos taken of the same subject but with different aspect ratios.