While the video uploads, only lower resolutions (like 360p) are processed – this makes uploading faster. Higher resolutions continue to process after your upload is done. Once they’re ready, you’ll see them on your video. More info here: https://yt.be/help/3QZ9 . Hope this helps.
A few factors can determine your video's quality – internet speed, player size, etc. YouTube may also adjust the video quality to help improve viewing conditions. You can try to change it yourself from the menu (settings) then select quality: https://yt.be/help/JXPZ