On August 18, Marriott will combine Marriott Rewards, Ritz-Carlton Rewards, and Starwood Preferred Guest (SPG) into one loyalty program. The combined program will not have a name until 2019, so you'll continue to see references to today's program names until then. However, they'll share the same structure for earning, redemption, and benefits.

My friendly advice would be to wait a few days (probably until August 19) and then read up about Platinum Elite benefits on the official Marriott website. Marriott has traditionally done a good job describing Elite benefits. The current content will be obsolete in a few days, so I wouldn't bother with it now.

Marriott officially has 30 brands, including the brands from its acquisition of Starwood in 2016, such as Westin and St. Regis. One of Marriott's luxury brands, Bulgari, does not participate at all in the loyalty program. Otherwise, benefits vary by brand. Some benefits also vary by geography or property type (hotel vs resort).

The short answer about Platinum breakfast is "it depends." With a few exceptions, brands will provide breakfast in some way (or at least daily credit toward the cost of breakfast).