Assigning content to a QR code or NFC tag occurs within the Lot screen where you assign what happens to a particular marker.

In short, you assign a Tag to a Lot, then you assign an Experience to that Lot  and when the Tag is scanned, the content specified for the Tag is shown.

For more information see:

Creating a Lot

Assigning Tags to a Lot

Creating an Experience

Assigning an Experience to a Lot