How does apache set the default document order, index.asp index.php index.html, which file to modify?

There is a conf folder in your apache, and there is a httpd.conf

in

it paragraph. It looks like this
<Directory />
    Options FollowSymLinks
    DirectoryIndex index.php index.html
    AllowOverride All
    Order deny, allow
    Allow from all
</ Directory>

This is to find index.php first, not index.html. . .
You can put them one by one
. After finishing, restart apache. It’s better to make a backup before changing, otherwise apache will be dead if it can’t get up.