Vertical or cropped?
A vertical original keeps its full frame and resolution. Cropping 16:9 to 9:16 keeps only the middle 32% of the picture, so a 1080p landscape clip becomes about 608×1080, which phones then upscale. Crop from 4K sources where you can (about 1215×2160 remains), and pick shots with one central subject.
Safe zones
Platforms overlay the caption, buttons and the account name on the bottom and right of a vertical video. Keep the subject and any text in the central area of the frame, and choose clips whose action doesn't happen at the very bottom.

