Youtube and Twitch expect a video with 16:9 aspect ratio, so you should send such a video. 16:9 is the ratio of the x- and y-resolution. So if you have some desired y-resolution, you can compute the corresponding x-resolution. If your desired y-resolution is 1080, multiply by 16 and divide by 9 to get the x-resolution: 1080 * 16 / 9 = 1920.
There are some magic standard resolutions Youtube downscales every video to. These are called 2160p, 1440p, 1080p, 720p, 480p, 360p, 240p, 144p and correspond to the y-resolution. Everything between these y-resolutions is downscaled to the next lower y-resolution, i. e. if you send a video with 1088x614, it is downscaled to 480p, which is 853x480, so your slightly higher resolution is lost. You do best, if your y-resolution corresponds exactly to one of the magic resolutions. Youtube doesn't down- or upscales to the nearest resolution, it always downscales to the next lower resolution, so if you send a video with y-resolution of 1079, it is downscaled to 720p.
See also:
https://support.google.com/youtube/answer/6375112?co=GENIE.Platform=Desktop&hl=en