Mortgage brokers aren't lenders, like the banks are. Instead, they provide an origination service, with the mortgage loan serviced by a traditional lender such as one of the major Canadian banks. Instead, brokers are a distribution channel for lenders to reach consumers like you.
Brokers often post lower mortgage rates than banks because they have access to multiple lenders and products and can ‘shop around’. Brokers also exploit their bargaining power to achieve volume discounts.