16:9, 9:16 and 1:1 are not simply three crops of the same frame. Each format changes composition, pacing and how much information can remain visible.
16:9 for web, YouTube and presentations
Horizontal video leaves room for people inside a wider environment, side graphics and a comfortable view on computers, TVs and projection screens. It is usually the strongest master for web, YouTube and presentation use.
9:16 for Reels and Shorts
Vertical video needs a different selection. A face, product or action must remain readable inside a narrow frame. A strong wide shot can lose all context when cropped. A useful 9:16 version therefore needs its own framing, caption placement, pace and opening hook.
1:1 for a compact feed format
Square video can work when a piece needs to stay compact in a feed and the footage does not support a clean vertical cut. It is not a universal answer, but it can suit a product, a statement, a simple promo or archive material.
Decide before the edit
When all formats are known from the start, shots and graphics can be built to survive every version. Asking for 9:16 only after the 16:9 master is approved often means reopening every shot. Platforms and ratios belong in the brief, not at the end of delivery.