Bookmarked https://fullsiteediting.com/lessons/creating-block-templates-for-custom-post-types/.
That explains! (I’ve been experiencing some issues with child themes, and was looking for a way to instead use the Site Editor only.)
[D]evelopers and users are expecting to be able to create this type of template in the new Site Editor. This is a reasonable expectation. Unfortunately, the feature is not complete yet and not included in WordPress 6.0.
