I have built a lot of these and watched more from the outside. The ones that travel share a structure. The ones that don't are usually beautiful.
Condition one: a one-sentence mechanic
"You can make a beat with the album."
That is the whole pitch, and it is the thing that actually gets forwarded. Not the URL, the sentence. People send each other descriptions, and the link is attached to the description.
If explaining your build requires a paragraph, it will not spread, because nobody forwards a paragraph. This is worth testing before any design work: say the mechanic out loud to someone who has not seen it. If they say "wait, so what does it do", go back.
Condition two: a personal artifact
The visitor has to leave with something that is theirs.
This is the difference between a site people admire and a site people post. Admiration produces a bookmark. Making something produces a screenshot, and the screenshot has the person's own work in it, which means posting it is not an ad for you, it is a post about them.
The practical implication: any build where every visitor sees an identical output has a ceiling. Randomness helps a little. Genuine input helps enormously.
Condition three: filmability
Almost all sharing of interactive work happens as a vertical screen recording, not as a link.
That should change the composition. The interesting part needs to be in the middle of a nine-by-sixteen crop. It needs to be legible at thumbnail size. And something satisfying has to happen within about three seconds, because the clip is competing against everything else in a feed.
Designing for a desktop viewport and then checking mobile at the end produces builds that are unfilmable, which means invisible.
What quietly kills a build
A loading screen with a percentage. People leave. Get the critical path small enough that the first frame arrives fast, and stream everything else behind the experience.
An onboarding sequence. Every instruction screen is a place to drop out. The mechanic should be discoverable by touching it. If it needs explaining, the mechanic is too complicated.
Sound muted by default on a music build. Someone who tapped a link from an artist's announcement wants sound. The web-wide default of muting is correct for autoplay video ads and wrong here.
Scene two. The single most common budget mistake is the second and third scene that most visitors never reach. One mechanic, built exceptionally, beats three built adequately, every time and by a lot.
Desktop-first performance. A build tested on a fast laptop on office wifi has not been tested. Test it on a three-year-old Android on cellular data. That is the actual device.
Distribution is part of the build
Who posts it first is a build decision, not a marketing afterthought.
The first fifty posts determine whether the next five thousand happen. Those fifty should be planned while the thing is still being designed, because if you wait until launch day to think about it, you are relying on luck, and the builds that appear to get lucky almost always had someone lining up the first wave.
Craft is necessary and insufficient
I have seen technically remarkable builds go nowhere because there was nothing to post, and simple ones travel for a week because the mechanic fit in a sentence and the output belonged to the person who made it. Decide the mechanic before anything else gets designed.