Same way you get to be any other kind of nerd - by getting really interesting in something.
In my case, I'd had some interest in calendars for a while, ever since I came across this converter. I'd also always wanted to learn programming, but my previous efforts just kind of fizzled out.
In 2020, I had some COVID-related downtime, and decided to use it to learn programming again. I worked my way through a book on Python, and when I finished, decided that the best way to practice was to write a real programme. I figured an extensive calendar converter would be a good way to learn, since it's all just maths. Writing a converter involved doing research to learn how various calendars work, writing code, and comparing it to existing converters and historical data. This in turn led to finding out about more and more obscure calendars, and I just became obsessed with tracking down vague hints and references.
The Chinese calendar in particular took me about a year of admittedly sporadic work, plus a lot of frustration, to figure out, because while there are plenty of descriptions, most of them are poorly-worded and not very descriptive. I also ended up having to write a whole library in Fortran to calculate the position of the sun and the moon. Yes, Fortran.
Right now I'm trying to figure out how traditional Indian calendars work, which is a whole challenge because they're even less-well documented than the Chinese calendars, and what documentation I can find tends to result in dates that don't match official dates.
I've put my notes on the calendars I've so far implemented here, with sources. Some sources are sketchy, but were all I could find.