Where is root htdocs folder for gateway?

Suppose, I have running gateway at domain https://ipfs.example.com

This works fine when I use is as gateway. But I want to change the content served at this root domain viz. /. How to change that content. Where is that folder?
Currently, this root domain gives 400 error

Any light here?

There’s a RootRedirect option in the configuration (go-ipfs/config.md at master · ipfs/go-ipfs · GitHub).

But what I do is to run nginx in front and serve / directly without involving the gateway.

What will be the content inside " " at RootRedirect? Is it just local folder address or localhost fqdn?

Btw, nginx setting on '/' path is not working and giving 500 error for valid gateway urls too.

@lidel plz help. Official documentation has no further guidance there.