A Platform business is a business that give away the service/product (primary) at below cost or loss (loss leader) but the actual business model is the make money by complimentary products and services being sold (upsold) to the customer.
Examples of Platform Businesses would be Payments Companies (e.g. Google Pay etc.) where the platform's core business (i.e. facilitating payment) does not make money (loses money on the contrary) for the parent organization but it generates tremendous amount of data of the customer (payer) and the seller (receiver).
Using this data generated the platform business can do any number of things around this business model (e.g. lending to merchants, credit cards to users etc.) which would eventually generate enough cash to sustain the loss making payments business.