How to combine or merge videos into one on any device
Most camera rolls can play clips one after another, not merge them into a single file. Here is what actually joins them.
Why this needs more than the camera roll
Playing several clips in sequence in the Photos app, or grouping them into an album, is not the same as merging them into one video file: the clips stay separate, so sharing them as a single file, or uploading them somewhere that only accepts one video, still doesn't work.
On an iPhone or Android
Neither the Photos app nor the default camera roll has a built-in way to merge clips into one video file. iMovie (free on iPhone and Mac) can do it: add the clips to a new project in the order you want, and export. This is a full video editor, though, for what is otherwise a simple join.
On a Mac with iMovie
Same approach as iPhone: create a new project, drag the clips onto the timeline in order, and share/export as a video file. Reordering clips is done by dragging them along the timeline before exporting.
On Windows
The Photos app's video editor (Clipchamp, built into recent versions of Windows) can combine clips on a timeline similarly to iMovie: add each clip, arrange the order, and export. Like iMovie, it is a general editor rather than a single-purpose merge tool.
A tool built just for joining clips
Stayput's combine videos tool skips the full editor: drop in the clips, drag to reorder them, and it joins them end to end into one MP4, keeping the original sound, directly in the browser with nothing uploaded. It handles MP4, MOV, WebM and MKV clips together in one merge.
Questions
Can I merge videos without a full video editor?
Yes, a tool built specifically for joining clips end to end skips the timeline, transitions and other editing features a full app like iMovie or Clipchamp includes.
Can I merge videos in different formats together?
A capable merge tool can combine MP4, MOV, WebM and MKV clips in a single output; some simpler methods require all clips to already share the same format.
Does merging videos re-encode and lose quality?
Some quality loss is common since the clips need to be combined into a single continuous stream, though a well-built tool keeps this minimal at a matching resolution and bitrate.
Can I reorder the clips before merging?
Yes, on every method above, whether by dragging clips along a timeline in a full editor or reordering them in a dedicated merge tool before joining.