Open a url from a website in installed App

I have a Website which contains a few URLs to different websites (Yammer.com, MS Planner and more). The site is a kind of portal for User to access different sites from this portal. I reworked the site and made it respsonsive.

So the next step is, to open the url in a native app installed on the Android or iOS Device. But I have no idea how to do that.

I read a little bit about Deep Linking, but in my understanding it's used for Apps.

How can I setup my URLs or website that they open in the app if they are installed or in the browser if the app is not installed?

I'm new to webdevelopment so please, don't hate me for the question.

Best regards wit4r7