Depending on the actual content you're viewing, you might not want it to actually fill the entire screen. Your Note 10 has a fixed resolution for its display and when you're watching something the through that XBox app the actual content is probably going to be a different height x width ratio than your Note's display. If you use that app in portrait mode and then in landscape mode the h x w difference is more apparent and obvious. Black bars often compensate for empty space needed as the content you're viewing will often not be the some h x w ratio as the display, otherwise the image would need to be distorted to actually fill the screen

These numbers don't actually correspond to an actual phone screen but just for simplicity, if your Note had a 20 pixel height and 12 pixel width, and you're watching streaming content that's 4 pixels high and 3 pixels wide, it's a matter of either scaling up the height by x5 or the width by x4 in order to maintain the exact same height x width ratio. It's one or the other. Scale both up independently and then that does fill the screen but distorts the image as the ratio is changed; do one or the other and then the result can be either black bars used as 'filler', or the height or the width of has to be cropped out. Most people find highly distorted images to be annoying, and cropping off top/bottom or right/left images can work on a limited basis, generally just using something like black bars will maintain image quality.

 

Are all those '...other phones...' you're referring to also Note 10 phones? Different phone models will have different screen resolutions and different aspect ratios, there is no single standard for all phones to use. Some phones have a lot of system resources for the display quality to be a priority, others not so much.
https://www.gsmarena.com/samsung_galaxy_note10+-9732.php
https://www.samsung.com/us/mobile/galaxy-note10/specs/
If all those other phones are indeed Note 10 phones, find of out what the display settings those other phones are set at, and check if there are any display options in there Xbox apps to use in yours.

Keep in mind that height and width aspect ratio issues are all based on math
-- To make a 4 x 3 image to fill a 8 x 6 display is just a matter of scaling up the height and the width by 2. To do the same with a 12 x 6 display, a matter where the aspect ratio is much different, than it involves a different approach, it's an either/or matter. Scale up by 2 to fill the display's width or by 3 to fill by height.
-- Scaling both up by 2 will result in that same 4 x 3 image appearing as 8 x 6 on that 12 x 6 display.
-- Scaling both up by 3 will result in that same 4 x 3 image appearing as 12 x 9 on that 12 x 6 display.
The former fills the display height but there's 'dead space' that needs to be filled (often black bars) to fill the width.
The latter fills display width but requires cutting off/cropping of the image's height.
-- Scaling both independently from each other is a problem too though. Scaling the image width up by 2 will fill the display width and scaling the image height up by 3 will fill the display height, but this alters the image's aspect ratio so it will result in a distorted image (stretched out in one direction).
https://en.wikipedia.org/wiki/Aspect_ratio
The basic issue is every image/video will have a set height x width so its aspect ratio is also set. Every display, be it in mobile device, a desktop, a laptop, a TV, whatever will also have its own, physical height and width. The aspect ratios between stuff we're viewing and what we're using to view them are in countless combinations that rarely involve a one to one multiplier. So image content is always going to involve a lot of 'fudging' where the image aspect ration has to be manipulated to fit into whatever screen we might be using. Cropping off at the top/bottom and/or right/left along with different height and width scaling to slightly distort the image is a typical thing, it just has to be done in moderation or the image quality is affected.

 

Xbox was designed to work with 16:9 aspect ratio TVs. Which the Note 10 isn't, so that's probably why there's black bars. The alternative would be to squash or crop the 16:9 Xbox video, to make it full screen on a display that isn't 16:9.

 

Check the app settings maybe? Other than that, no idea, as I can't use Xbox live myself. I've only seen Xbox been played on a TV.