whats the easiest way to write big config file?

I think we need more context here. What problem are you trying to solve? What does "big" mean? (I'm sooo proud of myself for not saying "a big file editor".)

so one of the application has hundreds of config and most of them are same across multi regions. so i was wondering if there is any way to use common config file

There's a couple of issues open for that sort of idea, but nothing implemented yes as far as I know. There are two options that have been recommended more than once in the past.