What is your model Apple TV box?
Identify your Apple TV model - Apple Support
Find your Apple TV model number - Apple Support
What is its version system software?
constantly buffering
Somehow that will be related to network bandwidth. There is no other reason for buffering.
Could it be that you are on the edge for what various services need? Netflix, Hulu may need less bandwidth than Apple TV app Store/Library content, as third party content providers typically stream using lesser bitrate.
Please measure the bandwidth to the Apple TV box using the Speedtest app on the Apple TV box. 4K HDR may need up to 25 Mbps. 1080p SDR may need up to 8 Mbps. If you have multiple simultaneous users in your household on the same internet connection, then the bandwidth gets shared as well, effectively lowering what is available to the Apple TV box.
Try restarting your Apple TV box, your router, and any other powered network gear that you may have, for a fresh connection attempt with optimized bandwidth properties.