IBKR does some of its trading off-exchange like the PFOF types, but they do it in-house, not on a paid-for basis. If you see "SMART" in the routing of an order on IBKR pro, often that order went to one of their darkpools or was filled by them off-exchange (at a price better than the NBBO).

The price improvement per share also sometimes comes from exchange rebates that they pass through to customers.