How to choose different list design/layout/style in SharePoint online?

I'm maintaining a modern SharePoint online site and have a backup test site. I have implemented a list on both sites in the same way, however the design/layout/style is very different.

On the official site the design/layout/style is classic:

On the backup site however it's more mordern:

I would like to have the modern look on both sites. I wasn't able to find out what triggers the different designs/layouts/styles and therefore can't change it. How can I achieve this?

Thank you in advance for your responses!