Of course I can't speak for anybody else, but from my experience it depends on the kind of DBA and the kind of company.

The larger the company is and/or the more experienced the DBA'er(s) is, the more likely it is there will be some kind of repository built. Sometimes this will be the same as the ones you can find on the internet, but most of the times this will be an adjusted version. Depending on your environment, most of the times the code you find on the internet is not exactly wat you want/need. So you end up changing and adding stuff to fit your needs.

In my previous job (large environment, >250 instances) there was a complete monitoring system built from scratch. They start building it in the year 2000 and up untill today the monitoring system is still being improved and expanded.