MathematicalPhysicist said:
But I cannot find the time to write articles in Wiki, so I wondered how did those volunteers find the time?
Are they pensioners?
All age groups.
You found the time to write thousands of posts here, many of them answering questions others had. It's not that different from writing Wikipedia articles.
Articles (longer than a few sentences) are rarely written in one step, the neutrinoless double beta decay is an outlier in that aspect. Usually articles are improved in many incremental steps by multiple different users.
The history of "Lepton number" is a more typical process. The article was
started in 2004, with just four sentences. Then
someone expanded it two months later, and another month later
it was expanded again, then tons of minor edits changed a word here and there, rephrased something, changed the formatting or made other smaller edits, and over the next 17 years the article has grown a lot and most of it was changed at least once.
Who has the time to fix a typo or improve a sentence in an article? Everyone.
In many cases that doesn't even need expertise in the specific topic.
MathematicalPhysicist said:
How is it decided which edit stays and which one doesn't?
Democracy or Dictatorship?
Consensus, ideally.
For most edits the key principle is called BRD: Be
Bold with the edit,
Revert if it's bad,
Discuss if needed. If you change an article and no one objects then that's the new version. If it is reverted to the older version keep that older version or start a discussion. Usually this discussion will find a version everyone is happy with.
If that doesn't work there are several ways to give the discussion to a broader audience, which usually leads to a version most people are happy with. In some extreme cases a good solution isn't possible, the default option is to keep the status quo. As an example, aluminium vs. aluminum will never have a solution everyone likes, the article is
aluminium and it's unlikely to ever change unless that spelling falls out of use.
Aluminum does get you to the same article, of course (it's a redirect).