3.10. How does commercial Qt licensing work? Do all my developers need to have a valid Qt license?

There is a whole FAQ section for this topic: https://www.qt.io/faq/tag/qt-commercial-licensing 

Each Qt user must have their own assigned Qt commercial license. UI Designers, Technical Artists, Software Developers, or Test Automation Engineers might have different kind of Qt licenses, but every single person needs a valid subscription license.