Anytime you create a new "app" on this platform it is automatically a mobile-first website. It will be fast, offer flexible functionality, and if you want you can turn on SEO features and make your website visible to search engines. It will also display well on desktop.
With the flip a switch, you can also turn a mobile website into a progressive web app that will prompt a user to add it to their home screen (app icon, short name and all).
When made an app, your website will centime to function as a search-friendly website so you will be getting the best of both worlds.