[configuration] How to set index.html as root file in Nginx?

in your location block you can do:

location / {
  try_files $uri $uri/index.html;
}

which will tell ngingx to look for a file with the exact name given first, and if none such file is found it will try uri/index.html. So if a request for https://www.example.com/ comes it it would look for an exact file match first, and not finding that would then check for index.html