I don't think the 777X is actually the newest Boeing plane (in april 2020), since it's only a new variant of a previous Boeing plane. So the actual newest plane would belong to the 787, since it's the newest plane. If that makes sense? Can anyone answer why Google says the 777X is the newest Boeing plane?
While the 777X is a derivation from the older 777 Classic family of airplanes, it is, in fact a brand new airplane, similar to the difference between the 747-8 and the 747 Classic line or the F/A-18E/F Super Hornet compared with the F/A-18A-D series. Based on that and the rollout date of the 777X, it would be BCA’s latest airplane.
One can refer to general types, variants of a type, and individual machines or airframes of a variant/type. Any example of each may be distinguished from other types/variants/machines as a different plane. Which of these is indicated by "plane" depends on context. For example de Havilland DH.88 Comet G-ADEF was a newer plane than DH.88 Comet G-ACSS. Or, the Spitfire Mk. VIII was actually a newer plane than the Mk. IX. But which was the newer plane, the Comet (which flew first) or the Spitfire (which was designed first)? The context is unambiguous in all cases, you know what I mean. Since the Google context is equally unambiguous, there is no problem.