To download a Ugoira, use the "Save as video (right click and save)" link beneath the image. This is a WebM conversion that can be played on regular video players, but is not lossless in quality.
To upload a Ugoira, paste the Pixiv URL into the source field on the upload page, or upload using the bookmarklet. Uploading a Ugoira file from your computer will not work.
Ugoira is a custom format exclusive to Pixiv. Animations from other sites cannot be Ugoira.
Ugoira is often considered inferior to animations found on other sites due to Pixiv's limitations on uploading GIFs or MP4 videos. As a result, artists may convert their animations to GIFs before uploading, introducing dithering and/or color banding, and Pixiv's recompression process can introduce additional JPEG artifacts, affecting the overall quality of Ugoira animations.