How to switch Unity assembly context in Rider

Background/Situation

Rider integration with Unity allows generating additional project files for various purposes. Example use case:

When editing code inside preprocessor branches, only theUNITY_EDITOR branch is active, other code is greyed out, and neither Code Completion nor Code Analysis is available for this code.

Question

How to switch editor context to a different project?

Answer

In the top right corner of the editor window, click the selector with the assembly name and select the appropriate assembly: