People on other phones are getting full screen though.
My Xbox itself is in 4K. I have my phone set to WQHD+ 3040x1440.
Is the default DPI on the note 10+ 498?
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.