I’ve always found this a little confusing as well. I’ve looked into it a ton, and I still don’t really get it, but what I can tell you is that designing UI for a mobile app is completely different than designing UI for a website or desktop app, you have to go into it with a different mindset, because mobile UI design has it’s own whole new set of rules. Since there are so many different types of phones with their own dimensions and resolutions, you have to design something that can work for all of them. I think the best thing to do at this point is to figure out what the highest phone resolution is right now, and the most common aspect ratio, then just design your UI within that.
With that being said, I would also appreciate it if someone could give a better explanation of how to design UI for mobile.