How can i add a custom page and add it to the header

Go to the landing page and more → Click on ‘Create a new page’.

Click new blank page

Create a name for the page (try to make this page’s name unique for its slug).

Afterward, you can design the page as usual. After publishing the page, please copy the link of the page from the online store → view page (in my case, it is https://quandoan21-otc-6.myshopify.com/pages/about-us-on-header).

Go to the theme editing section and replace the link you just copied into the navigation button on header.