The QR codes are just QR-encoded URLs (run them thru any QR decoder, or copy-paste them from the app into a text file). You can paste a URL into the client GUI instead of scanning a QR code, or use them in code.
The invite URLs are actually just encoded OpenPGP key fingerprints used to set up encryption, dressed up as delta.chat URLs so that Android will let Deltachat get them (by pretending to “open” them), and confused users can easily find a help page.
More here: SecureJoin Protocol, other standards used in DC.
But yeah, making stuff up works. 