Repeat Video.

Cross-fade the join. Set the number of plays, then pull the cross-fade up from zero: the last fraction of a second of each pass dissolves into the first fraction of the next, so instead of one frame following a frame it was never next to, there is a short dissolve where both are on screen at once. A quarter of a second is usually enough on handheld footage and half a second suits a locked-off shot. The cost is length, and it is stated: each join plays its overlap once rather than twice, so three plays of a 2 second clip with a 0.4 second fade lands at 5.2 seconds rather than 6.0, and the readout shows that while you drag. The audio crosses over in the same place, so the sound does not click where the picture no longer jumps.

Because a loop has to be able to run backwards, and a video decoder cannot. To bounce a segment, or to cross-fade the end of one pass into the start of the next, the tool needs more than one frame available at once, so it decodes the segment a single time and keeps every frame of it as a compressed still. Thirty seconds at 30 frames a second is 900 stills, which is a few tens of megabytes and comfortable in a browser tab; a few minutes would be gigabytes and would take the tab down with it. The cap moves with the frame rate rather than being a flat number: a 60 fps source gets 15 seconds, because that is the same 900 frames. If your moment is longer than that, trim it first and loop the trim.

Yes, and by exactly the amount you set, once per join. A cross-fade works by overlapping two passes: the last 0.4 seconds of one pass and the first 0.4 seconds of the next are the same 0.4 seconds of output, played once, dissolving from one to the other. So three plays of a 2 second clip with a 0.4 second fade is 6.0 minus 0.8, which is 5.2 seconds, because three plays have two joins. The readout does that arithmetic while you drag the slider, so the number on screen is the number you get. A boomerang has no join to fade and so loses nothing.

It can, and usually it should not. The back pass plays the audio backwards, which is what a true palindrome means, and reversed speech is gibberish while reversed music is a novelty that gets old inside one bounce. So the tool preselects silence when you switch to boomerang and tells you why rather than quietly stripping the track. If you do keep it, the audio period is matched to the picture period frame for frame, so the sound turns exactly where the picture turns instead of drifting out of it.

Yes. The output is an H.264 MP4 with AAC audio and a key frame every two seconds, which is the file both platforms want and the one every phone plays without thinking about it. Neither of them needs a special loop flag: a short MP4 in a feed repeats because the player repeats it, so what matters is that the first frame and the last frame sit next to each other cleanly, which is what the cross-fade and the boomerang are both for. A boomerang exported silent will not trip a music claim either, which is the other reason to leave the sound off.