MP4 is a container, not a quality setting
Think of MP4 as a box that can hold a video track, an audio track, and sometimes other information. Two MP4 files can use different codecs and look very different. The file extension alone does not tell you the resolution, bitrate, or whether sound is included.
Resolution is only part of the picture
720p and 1080p generally describe the frame dimensions of landscape video. Portrait and square videos may use different dimensions while being grouped under similar quality labels. Sharpness also depends on the source, compression, motion, and bitrate. A larger file cannot restore detail missing from the original.
Why a download sometimes has no sound
Some platforms deliver video and audio as separate tracks. A video-only file can play correctly while remaining silent. Choose an option that explicitly includes both tracks when the source provides one. Combining tracks may require merging them into a compatible file.
Pick for the job, not the biggest number
For a quick reference on a phone, a smaller file may be enough. For your own editing project, keep the highest-quality original you have permission to use. Converting or recompressing a file can introduce quality loss; keep an original copy before experimenting.
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