I would think that tomato is an adjective describing the kind of soup, not the quantity. So is this plural because the soup is made with many tomatoes?
It's not an adjective, it is an "of" relation, I don't remember the English grammar term for this. It is a soup of tomatoes. There is no agreement in "of" relation.
It changes with different foods. I don't know why
for example we say מרק בצל (onion)
No, but tomato is feminine. So because it needs to be made plural, the feminine ייה ending becomes יות.
Hebrew works the opposite way German does. In the construct state, the first word defines gender. It's German which has the compound noun's last noun define gender.
I was struggling with whether tomato would be singular or plural. I wonder if there is a של in there that is assumptive -- i.e. מרק של עגבניות (i.e. soup of tomatoes)?
I don't have this very well understood myself, but I know that we forgo shel because the nouns are put into a construct state. It basically means "soup (of) tomatoes". What I'd like to know is if the noun 'tomato' was put into plural because of the phrasing I just posted, or if it changed to conform to the rules of smikhut (construct state).