Thanks for the interesting article! After reading it, two questions arose in my mind:

Does this mean that we'll not be able to use PHP so much like we do now? I mean that every template is currently based on PHP, while FSE blocks are HTML based.

When I look at the example in the Codex (the link Ari shared), I see a functions.php (only used to set up theme), I see a index.php (just a fallback), some .html template parts (no PHP here?) and an Experimental-theme.json (also no PHP here?).

Will this mean that files like single.php, archive.php and home.php will become redundant? Or is my understanding of FSE themes incorrect?

Second, what will this mean for the customizer? Where is the customizer in FSE themes and how will it function?
Will it be possible to link values of customizer fields to the JSON file with the global styles?
And what about things like get_theme_mod() for placing values of text controls inside templates?
Or are those things currently being worked on?

And perhaps a question for Ari, since he's a (former?) Kirki developer: will Kirki play a big role in things like creating the global styles in the JSON file? I would imagine sth like Kirki generating the JSON file in a similar way like how we now add customizer fields?

Thanks! Ralph