A landscape photo set as a wallpaper gets cropped to a tall shape by the phone, and the phone picks what to keep.
Phone screens are around 9:19.5. Photos are 4:3 or 3:2. Nothing about those shapes matches, so a huge amount gets cropped, and left to the phone that crop is usually from the centre with no regard for what mattered.
Keep the middle of the frame clear. Icons occupy the top and bottom, and the lock screen puts a large clock across the upper third. A face or a focal point placed there will be permanently covered.
Do not enlarge a small photo to fit. Stretching a low resolution image across a modern phone screen looks soft and blocky, and phone screens are dense enough to show it clearly.
Modern phones are about twice as tall as they are wide. None of the preset shapes is that tall, so choose Free and drag the box until the height is about twice the width. For example 1080 by 2340 pixels is a typical full screen wallpaper size.
Keep the subject in the upper middle of the picture, because the clock sits at the top and the app icons cover the bottom. Check the pixel size shown, since a very small crop will look soft once it is stretched across a phone screen.
Match your phone's resolution. Around 1170 by 2532 suits most recent phones and scales acceptably on others.
Because the image is not the right shape, so the phone is cropping it to fit. Cropping it yourself first fixes that.
No.
Free. Drag the box so it is about twice as tall as it is wide, and watch the pixel size to make sure it stays large enough.