As others have mentioned, whether to use だ or not is not fundamentally about adding emphasis, that is actually really more a side-effect of various other factors.
In reality, ending with だ is technically simply the complete form of a full casual sentence, and so arguably should be seen as the "standard" construction. It is what most people are taught in textbooks, etc. However, it is the case that in the real world, many speakers, when speaking casually, will just tend to drop the だ in many situations. Because of that, when people would normally leave off the だ but instead choose not to, it can sound more forceful or emphatic as a result.
However, it's also not the case that everyone always leaves off だ when speaking casually either. Whether someone will choose to leave it off or not depends on the situation, and the sentence. In particular, だ is typically omitted most of the time when saying short sentences of only a few words, but it is more common to actually leave it on when speaking longer sentences (I think partly because it can serve as a useful "end of sentence" marker to help the listener sort out more complex sentence grammars).
So in the examples you quoted, because they are longer sentences, ending with だ does not necessarily sound "emphatic", because it's actually more normal to keep it on the end of the sentence in that sort of situation anyway.