The Apple TV app Library is using 16:9 poster art for all media now: movies, TV shows, and home videos; regardless of source aspect ratio.
For purchases from Apple’s media store, the artwork is set by the studios to specification from Apple. For older content without updated poster art, images with mismatched aspect ratios will get scaled down proportionally to fill the 16:9 area without cutting anything off.
For your own videos, you can set any image, but it will be displayed in a 16:9 frame. Images with mismatched aspect ratios will get scaled down proportionally to fill the 16:9 area without cutting anything off.
The Computers app (for Home Sharing) on the Apple TV box (tvOS 18) will use a black filler for mismatched aspect ratio.
The TV app on iPhone (iOS 18) will use a transparant filler for mismatched aspect ratio (with 16:9 drop shadow).
The TV app on Mac (macOS 14) will use an auto-generated color gradient filler for mismatched aspect ratio. [I can’t check macOS 15 at this time, but I suppose it will be somewhat like iOS 18?]