My recent example: Booked a ticket on Expedia that involved AA & Emirates. AA delayed their flight and it affected the Emirates flight. I 'figured' Expedia would just rebook on the later Emirates flight. Wrong. They said if I just requested the later Emirates flight, it is a voluntary change and I would have to pay fees

Instead, Expedia called AA (and I was only on hold, not part of the call). Expedia said AA would decide how THEY would fix it. Expedia really had nothing to do with it and I would've been better off calling AA directly (probably).

In this case, AA changed the flights, routing, and airlines. New itin was AA and BA, via LHR instead of FRA.