PNG: export the current frame
PNG saves the currently selected frame. With transparent paper, the result can be layered in another editor. Remember that it exports one scene, not the entire animation.
GIF: share short motion immediately
GIF packages frame order and timing into a looping image that needs no separate player. Its palette and partial transparency are limited, so it suits bold shapes and short loops better than subtle gradients.
Sprite sheet: move frames into another tool
A sprite sheet arranges every frame in equal cells inside one image. It is useful when another tool or game engine can import the known cell size and order. Timing is not stored in the image, so keep the duration values separately.
.loop.json: the editable source
The project file stores frame order, per-frame duration, paper, and editing information. Reopening it through Project Open preserves far more editing control than importing an exported image. When sharing it, explain that it contains working data rather than only a picture.
Keep an extra copy of both the project source and final result in another folder. Work is not automatically uploaded unless you choose cloud save, so browser data alone should not be treated as long-term storage.