How do some YouTubers make Siri talk on their iPhone?

YouTubers like Smosh somehow made Siri say what they want, which can't be done normally on Siri. How do they do that?

This is how I do it:

Have someone (or yourself from another number, etc.) text you the word(s) or phrase(s) you want to be said by Siri and then have Siri read back your messages. It should read it verbatim for you… And with a little bit of editing and audio overlaying, you should be able to reproduce what those YouTubers have.