I’m looking at booking a round trip Europe - US itinerary for next year and I always booked through Lufthansa until now (as their TATL flight times have suited me better). This time around, the Lufthansa flight search won’t show me the flights I want to take, but United will.

As I don’t need checked luggage, I’ve always booked the Eco light fare (Basic Economy on United). With Lufthansa, this allowed me to select my seat free of charge when online check in opened. Is this also the case with United (and does it depend on who operates the flight)? I can’t seem to find this information.

Also, who would I be checkin in online with, Lufthansa or United? Flights are operated by Lufthansa, Lufthansa, United on the way there and United, United, Lufthansa on the way back.