Joerg Sprave calls it the Sling-X-Bow, which is cool and also pretty descriptive. It’s a slingshot-powered crossbow that feeds from an integrated magazine.
What makes it special is that the forend doesn’t just cock the magazine, it controls the bolt feeding. It shouldn’t be too hard to reverse-engineer the design but if you want him to publish a complete guide and how-to video vote for him in the “German YouTube Oscars” here. You can vote once per day, bonus.
Best of luck Mr. Sprave, we’re all pulling for your crazy mad-scientist self.