What I miss in this book is the reasoning.

https://gibbok.github.io/typescript-book/book/differences-be...

So we know there are types and interfaces. One support declaration merging, one does not. Both can extend others, but in different ways. But why? Why there are two of them? When should I use them? Is one better than the other?

Determining the motivation for design decisions is outside the scope of what any external party can and/or should do. The most the author could do is echo what's in the docs (if present), or give their own guess on why things are the way they are.