Hi Harald,
You can direct URLs to open in the App by adding moodlemobile://link= before the link. When generating the QR code just make sure you add this before the URL, for example "moodlemobile://link=https://moodlesite.com".
Worth testing though as there will be limitations on where you can go depending on if the app supports viewing that page. I just tested on a chapter of a Book and it took me to the Book rather than that specific chapter.
Ian