According to the Marriott Bonvoy Loyalty Program Terms & Conditions, at Westin (and six other Starwood legacy brands, including resorts), Platinum Elite members and above can choose an Elite Welcome Gift from a short list:
-- “1,000 Points per stay”
-- “Amenity per stay”
-- “Breakfast in restaurant per night of stay for member +1”

It does not define the specific details of the "breakfast in restaurant." It doesn’t say "partial credit toward breakfast." It doesn't say “continental breakfast.” It doesn't say “cheapest item on the menu.” It just says “breakfast in restaurant.” It's reasonable to assume a full breakfast in line with what other guests are having.

Usually, full-service Marriott Bonvoy hotels (whether Starwood legacy or Marriott legacy) that provide breakfast in the restaurant for Platinum Elite and above fully cover the breakfast, hot beverage, juice, tax, and tip. When the breakfast is a buffet, it generally includes anything on the buffet, but some hotels only allow cold items unless the member pays an uncharge (which is okay if the cold buffet offers enough choices and quality).

I don't know why some hotel managers think it's smart business to give the middle finger to Platinum Elite members and above, in direct contradiction to the published benefits.

A daily credit toward breakfast is fine at Courtyard, AC Hotels, and Moxy, because that's the published benefit at those brands. It's not acceptable at Westin and other full-service brands with better published benefits.