Choose a Mac-friendly video container and codec
After the browser finishes, find the file in Finder's Downloads folder and open it with QuickTime or another player. A WebM or AV1 file may need a player or editing application that understands that codec, even though the download itself completed correctly. For editing, preserve the original source resolution and frame rate instead of converting repeatedly; each lossy transcode can reduce quality. 4K and 60fps options contain more data, so allow extra transfer time and disk space before moving the file into a project.
A Mac browser can save the selected media as a normal file in Downloads without installing a converter. YTMetadata lists the source's available resolutions and containers; it does not upscale a clip or guarantee that every video is encoded alike. When YouTube supplies separate adaptive video and audio streams, the server muxes the selected tracks into one output. H.264 video with AAC audio in MP4 is a practical choice for QuickTime Player, Finder previews, and many iMovie workflows, although codec support can depend on the macOS release.