Need Help with App / Web display settings

My goal for my app is to have a different display for the App and the Web version.

  1. For the app, I have it looking roughly how I want it to work, a list of options to select, when selected a page opens with the details about that selection.

  2. For the web I would like to display the first page as a “column” on the left hand side of the page, and if an option is selected the second page will “stack” on the right hand side of the first page, instead of opening up a new page.

I feel like I have seen web designs like this but are they using different pages or just adding widgets on the same page?

For Example WhatsApp mobile app versus desktop version.