After comments & the question edit…

The issue is one of aspect ratio - the relationship between the width & the height as a percentage.

A phone camera will take an image in approximately the same aspect ratio as a modern TV, 16:9.
A dedicated camera, on the other hand, will take images at 3:2 [not 4:3 as in my comment, sorry]. This ratio is still much narrower than the phone's image & so will always have black borders at the sides when viewed on a phone.

Historical answer from before clarification

The viewfinder only shows about 90% of the actual photograph that will be taken.

So, very roughy, you will see inside the red line in the viewfinder, but the actual photo will be the full frame…

Live View, on the other hand, will show the entire frame.

The issue is the aspect ratio – the relationship between the width and height.

Phone cameras will take images with approximately the same aspect ratio as a modern TV, 16:9. A dedicated camera, on the other hand, will take images at 3:2. (The main exception is Micro Four Thirds, which uses a 4:3 aspect ratio, like the old 'square' TVs from 20 years ago.)

This ratio is much narrower than the phone screen, so will have black borders at the sides when viewed on a phone.

In the following image, the red frame shows approximately what you see through the viewfinder, about 90% of the photograph that will be taken. The black bars shows the missing part of the image that would be needed to fill your phone screen.

The viewfinder only shows about 90% of the actual photograph that will be taken.

So, very roughy, you will see inside the red line in the viewfinder, but the actual photo will be the full frame…

Live View, on the other hand, will show the entire frame.

After comments & the question edit…

The issue is one of aspect ratio - the relationship between the width & the height as a percentage.

A phone camera will take an image in approximately the same aspect ratio as a modern TV, 16:9.
A dedicated camera, on the other hand, will take images at 3:2 [not 4:3 as in my comment, sorry]. This ratio is still much narrower than the phone's image & so will always have black borders at the sides when viewed on a phone.

Historical answer from before clarification

The viewfinder only shows about 90% of the actual photograph that will be taken.

So, very roughy, you will see inside the red line in the viewfinder, but the actual photo will be the full frame…

Live View, on the other hand, will show the entire frame.