Replacing the default content created while the site creation

How do you replace the default title for first comment?
Mr WordPress on Hello world!

I could not able to locate this in theme’s comments.php file

I want to edit the title ‘Mr wordpress on Hello world’ to Admin on -Site Name-

How can I replace the default text created with about us page?

This is an example of a WordPress page, you could edit this to put information about yourself or your site so readers know where you are coming from. You can create as many pages like this one or sub-pages as you like and manage all of your content inside of WordPress.

to

It is the page about your self.Feel free to edit the page

Some times I cannot able to locate certain content(As I mentioned above comment.php and default about us page) in core files when perforimg search .is there any way to find certain content or code overall?

I can only answer your first two questions as I do not know anything about multisites.

The default comment and content in the ‘About Us” page is not coming from the theme files, but rather the database. They are the default posts/pages/comments accessed by the backend not by the theme files. To get to this content and replace it login to the admin via youraddress.com/wp-admin
and locate the about page in pages > about us. The comments are under the “Comments” section. Simply delete the page or replace the content.