YouTube decides what a video is partly by its shape: a vertical 9:16 upload up to three minutes long is treated as a Short, while 16:9 goes to the standard player. Sending the right canvas is how you choose which shelf your video lands on.
Crop-to-fill scales your footage until it covers the target canvas and trims the overflow evenly, so nothing gets squished. Going from 16:9 to a 9:16 Short costs the sides of the frame: worth checking that the subject sits near the center before you flip it.
Resolution earns its keep here: YouTube tends to give 1080p-and-up uploads its more efficient codecs, so a proper 1920x1080 file usually ends up looking cleaner after YouTube's own re-encode than an undersized upload stretched to fit.