If permission is required, what permission is it, where/how is it obtained, and what core onward-travel evidence is required?
The permission is an airport transit visa. How to apply depends on the country of residence, and in most cases the application is made in person, between 6 months and 15 days before travel. The core evidence includes a signed application form, a valid travel document, proof of legal residence in the country where the application is made, a travel reservation to and from the Schengen area, and valid proof of legal stay in the final destination country (passport, residence permit or visa).
See recorded sources