Click the add file button:

https://storage.googleapis.com/replit/images/1552425746912_0e5d004a85aae04ac61e252009a43354.pn

And name it about.html or similar. Then you can write some HTML in it.
Then in your index.html, you can add

<a href="about.html">About</a>

to add a link to that page.
Please upvote if this helps :)

Make your other html page that you want to refer to. Then, use this code

<a href ="test.html">Hello World</a>

replace "test.html" with your file and then you can also replace "Hello World" with the link text.

Please help!