Both sentences are grammatically correct and are perfectly sensible and idiomatic in the right context. Neither sentence can stand on its own: in both cases, “it” needs an antecedent that must come from the context. There is no particular difference with respect to emphasis. Any statement that one of these two sentences is unambiguously more correct or more idiomatic than the other is just wrong.
Here are some plausible contexts:
— Where did you two have your first date?
— It was at a restaurant in Florence where we ate the delicious steak by the river.
This sentence is an answer to a “where” question. Where did [something] happen? At the restaurant described in the answer.
— What was your favourite place to eat in Italy?
— It was a restaurant in Florence where we ate the delicious steak by the river.
This sentence is an answer to a “what” question. What kind of place is [something]? The restaurant described in the answer.
the chapter starts off with using 'what' clauses to add emphasis.
If the topic was emphasis, it's possible that one of the intended choices was this sentence, with a different construction (but even so, the existence of this sentence wouldn't have made the others wrong):
— It was at a restaurant in Florence that we ate a delicious steak by the river.
This sentence is more standalone than the two choices given in the question, because here “it” does not have an antecedent that comes from the context. The antecedent of “it” is the second part of the sentence: “we ate a delicious steak by the river”. This sentence has the same meaning as the simpler sentence
We ate a delicious steak at a restaurant in Florence by the river.
Starting with “it was” puts some emphasis on the restaurant.
I changed “by the river” to be attached to “a restaurant in Florence” rather than “ate a delicious steak”. This isn't absolutely necessary, and a lot of variations are possible in the order of complements. There is a slight difference in meaning. With “by the river” attached to eating, the statement refers not only to the food but also to the ambiance in which the food was eaten. With “by the river” attached to the restaurant, it's an objective detail about the location of the restaurant. I find the straightforward sentence a bit underwhelming if it's describing an ambiance, but it wouldn't be wrong.