They sell flow to KCG/Citadel/etc like most all (every?) retail broker.
Interactive Brokers is the only way to go for an intermediate person who wants to speculate and have high-powered tools for a relatively low minimum ($10k I believe, $3k or so for younger people under 25?). They refuse payment on order flow, as does Fidelity, IIRC.
Like I said I'm out of that game (minus small speculation on BTC for currency hedging purposes). I've found the love of index funds and low expense ratios.