Consider the complexity of a single shift: hygiene regulations (HACCP), cashier reconciliation, allergen management in the deli, and the proper rotation of perishable goods. Each of these requires certification. Online courses allow a part-time student working the evening shift to complete a 15-minute module on "Correct Temperature Logging" via their smartphone in the break room.
If you do not work for REWE yet but want to learn the skills to get hired (specifically cashier skills), REWE does not offer public courses for this. You must look for third-party providers.
The primary function of Rewe’s online courses is invisible to the consumer but vital to the corporation. With over 3,500 stores and tens of thousands of employees, traditional in-person training is logistically impossible and financially prohibitive. Rewe has therefore embraced and Gamification via its internal portals (such as the "Rewe Akademie").
You can search for or "Kassierer Kurs" on platforms like Udemy.