The high-flow set-up has both a flowmeter and a valve that allows for adjustment of the FiO2 more precisely. If you look at the circuit, you'll see that it's connected to the wall oxygen source with the same high-pressure hose that is used for mechanical ventilators, both invasive and non-invasive. There should also be an oxygen analyzer inline that measures the actual FiO2. On the pole holding the humidifier, there will be a knob for adjusting the FiO2, with the oxygen flowmeter either behind or to one side.
Before the development of Optiflow and the other high-flow circuits, we didn't run oxygen at high flow rates because there was no way to heat it enough to make it comfortable, and the bubble chamber arrangement didn't really provide much humidification either. In some ways, high flow oxygen acts as a form of CPAP as the flow rate splints open airways. As the patient's lung function improves, the flow may be weaned to 1L/kg first, then the FiO2 titrated to maintain sats, before the flow is weaned again. Once they get down to 3LPM, which is comfortable for most patients, regardless of the FiO2, the switch to low-flow cannulae is made, with the FiO2 at 100%. The patient is actually getting somewhere around 40% at the alveoli due to some blow-off during expiration and some leakage at the nares.