Can’t really name examples off the top of my head, but I haven’t noticed a clear criteria over time that would explain why some programs become ‘safe’ or ‘official’
Is it just basically 1) the program is prominent enough to be looked at + 2) some benevolent potentate in the Arch/Cachy universe ended up hearing about it and rubber stamping it?
Not a complaint I’m just curious as to how it works since I am fully converted to Arch/Cachy now 
Please at least give a quick pass to a search engine or similar before asking a question. The wikis are also great places to look first in particular.
In many examples chances are that an answer already exists along with documentation.
We are not even anywhere near in control of the AUR so this would not be the best place to ask anyways.
But it is outlined in the .. ArchWiki page for the Arch User Repository. In the section “How to get a PKGBUILD into the extra repository?”.
To quote;