Usually, yes. There are some exceptions when the connecting flights both land and take off from the same terminal but it's not something you can count on.
I’m thinking to buy a ticket from a flight aggregator like Expedia or GoogleMaps. We are not planning to leave the airport. It’s just a connecting flight. Would we still have to go through EES?
god i remember those lines at frankfurt, absolute madness. we missed a connection once and had to sleep on those awful airport benches. if you gotta go through it again maybe book a long layover, like 5+ hours to be safe
European airlines would collapse if it wasn't something you can rely on. Do you have any idea how many people connect in Europe despite not being eligible to enter the EU (for example a lot of India>US/CAN trips)? ;) The main requirement is that there can't be multiple Schengen stopovers (e.g. you can't do DEL-VIE-FRA-YYZ) because that would require entry of course. But as long as there's just one Schengen stop and it's all booked on one ticket, airside transfers are possible at all major hubs. It doesn't matter whether it's the same terminal or not.
Usually, quite the opposite. Most of those large airports that rely on selling connecting tickets have non-Schengen to non-Schengen transfer.