Question about Apple TV+ (about streaming quality)

Mi Box is not listed as a supported device, but Fire TV Stick 4K is. List of Apple TV app-supported devices - Apple

Video should render in the resolution of your settings, up to 4K (if adequate bandwidth is available).

The Apple TV App on 4K devices streams in 4K as long as the internet bandwidth supports it. Find movies with 4K, HDR, Dolby Vision, or Dolby Atmos in the Apple TV app - Apple Support

On Windows, Android and even Macs the https://tv.apple.com website’s quality is capped at 1080p.

Some people are experiencing low quality issues when using the website however, but its not that the website is physically limited to 720p or anything like that. .

Because its not one of the devices Apple has certified yes. It uses the Chromecast software platform which is certified and supported by Apple, but the device itself is not part of Apple's supported line of devices. So it may or may not stream in 4K. You will need to try it to find out if it does stream in 4K. It probably will though. But again no guarantees.

See here: link-> List of Apple TV app-supported devices - Apple

AllanMedeiros wrote:

So does that mean that if I use any device like mi Box or firestick, it will get the maximum quality that it supports, whether full HD or 4K?

The FireStick 4K should show Apple TV+ content in 4K, again, as long as internet bandwidth supports it.

The miBox may not, though if its just identified as a Chromecast with Google TV it might, but no guarantees, since it's not an actually supported device by Apple. It may just be using the Chromecast platform which is supported.