There are two kinds of mythical 10x developers. Fake 10x developers, and legitimate ones. The difference between the two is hard to spot at a glance, and the fake ones rarely suffer consequences.

A 10x developer does things that bring 10x the impact to the development process. They level up junior developers around them. They write solid code that other devs use for reference. They share ideas that can be read by the entire development department. They write static analysis tools that catch bugs of the entire department. They write tools that bootstrap projects and let devs worry about writing code instead of reading asine config docs. They write libraries that the majority of the development projects can use, easily speeding up development by magntitudes. They are basically doing what the day to day dev is doing, but at a global level.

A lot of 1x devs could do these kind of activities, but don't. Can you blame them? A 10x developer does not make 10x the salary. And most of the above requires a lot of hard work. Worse, they may even be beaten down and reprehended for trying to do such activities, or ignored. It depends on the culture.

A fake 10x developer does a lot of things the 10x developer does, but poorly. They write lots of bad to mediocre code, claim the feature as done, collect the credit, and then let others pay off the debt while moving onto the next project. They write shared libraries, but do so poorly. Other teams consuming this code must work around their poor bug ridden API. They typically move up ranks fast, on the premise of getting lots of stuff "done" at fast rates. Their high rank protects them from criticism and makes whatever they do "right by default". They will likely move to another company and continue their destructive path while earning large amounts of money. They essentially "fail up".

Be very wary of any dev with a mythical reputation and always evaluate their work, not their legend.