In a follow-up discussion with Wolfram Media themselves, some clarifications (as of 2022-11-21) for posterity:

1. The recommended way to emit PDFs is NotebookPrint (Save-as-PDF).
2. There is currently no public way to automate the emission and assembly of the one-notebook-per-chapter inputs (as recommended by the WolframBookTools paclet) into a single PDF.
3. Likewise there is no public automated way to generate tables of content.
4. Likewise no public automated way to generated the "answers to exercises".
5. Likewise no public automated way to generate indices.
6. Likewise no public automated way to emit the sexy HTML/CSS/etc. as seen on Wolfram Blog posts.

In short, we must ourselves (currently) crawl that last mile. :/

I think using the book tools paclet. Never used it but seems a good place to start.

https://www.wolfram-media.com/resources.html