(I have also posted this question to SharePoint Stack Exchange: link)
I am currently in the process of getting up to speed with SPFx and have gone through the Extensions Getting Started guide of the official documentation and skimmed a fair portion of the other documentation pages, but one point that eludes me is how to make effective use of the .properties member that belongs to each extension type.
I'm aware of two ways to pass in a value for these properties:
Pass them in via the URL during debugging
Specify them in the elements.xml file (for Application Customizers)
The first seems to be only applicable during development. The second is very limiting because (a) It only applies to Application Customizers and (b) it means having a fixed set of configuration values for the entire tenancy.
What's further puzzling is that I have looked at some official and unofficial samples for field customizers, and all of them have a documentation line showing how to pass in config values via the debug URL, but no indication of how these values would be specified in an actual deployment:
I can see that client-side webparts have a rich feature set for defining and specifying configuration values, but I am at a complete loss as to how to configure the three extension types (Application Customizers, Field Customizers, ListView Commands) on a per-site or per-list basis. Can someone point me in the right direction?