Say I want to visit somewhere like Osaka for 2 weeks, and am flexible on dates the whole year. My priority is the cheapest and most direct flight. I don't care about dates. It obviously takes too much time to type and trial-and-error pairs of dates. How can I find out efficiently for free?
How can I view all of the flights between two cities across all dates? doesn't answer my question. You have to pay to use Expertflyer. ITA ranges just for one month, not the whole year.
Skyscanner shows the airlines with direct flights between OSA and the US. I checked Delta and JAL's route maps, and they have nothing direct between SFO and OSA. United's flight map (screenshot under) proves it does, but it doesn't advertise what dates. These KAYAK searches including OSA-SFA and SFO-OSA show direct flights.