You've ignored the part where the declarative template is immediately attached to its parent as a shadow root by the parser, and its siblings become the slot c

You've ignored the part where the declarative template is immediately attached to its parent as a shadow root by the parser, and its siblings become the slot children. You don't need to instance or register anything with JS there, just copy-paste the template (admittedly lame, but kinda the same repetition as SSR today, just less involved and with a shadow root).