Hi and thanks so much for your feedback!
- Not all code samples will have the “usual” header. It comes down to some formatting functionality in the publishing software. Where we reference more complex code in it’s “final” form from the codebase that is committed and available for download, you’ll see the header. Where we embed code directly in the prose, you won’t. If there isn’t a header, the surrounding prose text should make it clear what file you are looking at.
- I’ve done my best to review the errata you submitted, and addressed most of it.
As a general practice, PragProg authors don’t provide solutions to exercises, which is often down to time and resourcing constraints. As you can imagine, we have our hands pretty full writing and providing complete code for such a lengthy book, even as we continue to work hard to keep versions up to date as LiveView has evolved in recent years! We very much encourage readers to share their solutions here though 
Some of the things we don’t explicitly explain in the book–like the capture syntax, or query preloading–we choose not to explain in the interests of time and space and to keep the focus on the complexities of LiveView. Some prior knowledge of Elixir (and things like Ecto) is assumed.
Thanks so much again for your feedback!