Generally, when a QR Code with links to sites such as Amazon and Facebook is scanned, it automatically invokes the landing page to be opened in the appropriate app.
However, sometimes the landing page is opened in the browser itself.
The reason that it might happen is because of the scanning app.
Certain scanning apps decode the QR Code and redirect to browsers such as Google Chrome. Chrome detects the link and opens it within the required app.
On the other hand, there are some scanning applications that decode and open the link in their own browsers. These browsers sometimes do not detect the link and fail to invoke the app. So the link gets opened within the browser itself.