That's not correct.
ui_resolutions has nothing to do with what device resolutions are "supported".
It refers to what screen canvas sizes your code is assumed to be writing to.
If you write code that assumes a screen canvas size of 1920x1080, then your code is assumed to be writing to an FHD canvas. If your code assumes a screen canvas size of 1280x720, then your code is assumed to be writing to an HD canvas, and if your code assumes it is writing to a screen canvas size of 720x480, then it is assumed to be writing to an SD canvas.
So for example, if you want to display a rectangle in the center of the screen that occupies half the screen area, if your code assumed it was writing to an FHD canvas you'd use a 960x540 rectangle at [480, 270], for an HD canvas you'd write code to put a 640x360 rectangle at [320, 180], and for an SD canvas you'd put a 360x240 rectangle at [180, 120].
Your code can target any or all of the supported screen resolutions. I suspect that most Scene Graph channels target only one canvas size: either FHD or HD. If you want to write code that targets more than one canvas size then you need to do a lot more work. In that case, you'd need to write code to determine the display resolution currently being used by the user's Roku device, and write different code to lay out the canvas differently depending on which resolution is being used (e.g. using a different Scene specifying different display coordinates).
Whatever screen canvas size(s) your code targets is what you specify in the ui_resolutions manifest file entry.
When the channel actually runs on a user's device, the Roku looks at the ui_resolutions list and picks the resolution that it considers best for what the user's TV and Roku support.
If the user's TV and Roku resolution is not one of those specified in your ui_resolutions entry then the output is AUTOMATICALLY SCALED to fit on the user's TV screen.
For most cases, you can specify ui_resolutions=hd and write code for a 1280x720 screen canvas -- or ui_resolutions=fhd and code for a 1920x1080 canvas, and let the Roku handle automatic scaling for different display resolutions. If you care about your SD users (which Roku no longer does), then be sure to test the channel on an SD device to ensure everything scales to be readable without a magnifying glass and doesn't get the edges chopped off. However, I've yet to see a Scene Graph channel that meets that criteria on an SD TV. It's obviously not something that Roku QA are looking at -- although it looks like they do care if your keyboard letters fit in the grid though.
