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.
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.