Container, codec and what conversion means
A media file has two layers that are easily confused. The container — MP4, MKV, MOV, WebM — is the wrapper describing what streams exist and how they are interleaved. The codec — H.264, HEVC, AV1, AAC, Opus — is how the audio and video data are actually compressed.
This distinction determines whether a conversion is cheap or expensive. Changing container while keeping codecs is remuxing: the streams are copied unchanged into a new wrapper, which takes seconds and loses nothing. Changing codec is transcoding: decoding and re-encoding, which takes time and discards quality.
So "converting MOV to MP4" is often a remux, because both commonly carry H.264 video and AAC audio. Converting H.264 to AV1 is a genuine transcode regardless of container. When a conversion completes suspiciously fast, that is usually the reason and it is good news.
Generation loss and choosing settings
Every transcode of a lossy format loses quality, and the loss compounds. The second encoder cannot distinguish the first one's artefacts from real detail, so it spends bits preserving blocking and ringing. Converting a file back and forth between formats degrades it measurably each time.
Where a workflow requires several steps, keep a high-quality or lossless master and produce each output from it, rather than converting an output into another output. If you must transcode from a compressed source, use a generous bitrate — the second encoder needs headroom to reproduce what it was given.
Raising bitrate or resolution above the source never adds quality. Upscaling 720p to 4K produces a larger file containing exactly the detail of 720p, and encoding a 128 kbps audio track at 320 kbps stores its artefacts more faithfully. Match or reduce, and reduce only as far as the delivery target requires.
Compatibility, and why files fail to play
Playback failures are usually codec support rather than container support. A device that opens MP4 files may still refuse one containing HEVC or AV1, since codec support depends on hardware decoding as much as software. H.264 in an MP4 with AAC audio remains the most universally playable combination by a wide margin, which is why it is the safe answer for anything going to unknown recipients.
Subtitle and audio tracks are frequently lost in conversion. MKV supports multiple audio tracks, chapters and several subtitle formats; MP4's support is narrower, so converting MKV to MP4 can silently drop tracks. Check the output rather than assuming.
Hardware encoding is much faster than software encoding and produces somewhat larger files at equivalent quality, because hardware encoders make simpler decisions. For archival or size-critical work, software encoding at a slower preset gives better compression; for quick conversions, hardware is the sensible trade. Everything here runs locally in your browser, so large files are limited by available memory rather than by an upload.