Fast and lossless when the clips match
Clips recorded with the same phone or camera settings, or exported from the same program, share the same format, size and sound settings. Such clips are simply placed one after the other without encoding anything again. The result has exactly the quality of the originals and even long recordings are joined in seconds. The result line says No quality loss when this happened.
This also works for parts of a long recording that a camera split into several files, and for pieces cut out of the same video with the video trimmer.
Clips of different sizes and formats
When the clips differ, for example a vertical phone video and a horizontal screen recording, or a WebM and an MP4, every frame is drawn into one common frame and encoded again. Size when clips differ decides that frame: Like the first clip, or Like the largest clip. Pictures of another shape are fitted in the middle with black bars, never stretched.
The result is an MP4 with H.264 picture and AAC sound, which plays on phones, Windows, Mac and TVs. Firefox cannot create AAC sound, so in Firefox the result is a WebM instead. Clips without sound are kept silent for their length, so the sound of the other clips stays in sync.
Privacy and limits
All clips stay on your device; nothing is uploaded and there is no watermark. Joining with re-encoding uses your computer's video encoder and takes roughly as long as the clips play, less on fast computers. Very long videos need a lot of memory, especially on phones; there the fast, lossless way works best.