'maguro' is tuna and 'uni' is sea urchin
Since I live with a Japanese woman, let me tell you what I've learned from her about sushi. First of all, it is not true that Japanese people eat sushi three times a day.
Second, sushi has found a place in America which it does not have in Japan. Originally sushi in Japan was a working person's sandwich. Traditionally Japan was not a bread culture. Japanese rice sticks together. So you can make a ball or other shape out of it, stick something inside, roll it up, put some seaweed around it (nori) and have the equivalent of a sandwich to take in a brown bag to work.
You can put anything inside. Anything. Whatever suits your taste.
Americans think sushi means raw fish. That's just the way Tokyo people do it. People from other parts of Japan (Kansai, for instance) put cooked fish inside their sushi.
And a real sushi doesn't have to be a neat package. The two or three times a year we have sushi for dinner, we make some rice, put out a variety of fish and vegetables cut into small pieces, and some leaves of seaweed. Put down a leaf of seaweed, put some rice on it, put some other ingredients in it, roll it up however neatly or non-neatly you can do it, then eat (after dipping in shoyu -- soy sauce).
It doesn't have to be a mystery. Make up your own recipe out of ingredients you like.