Toyota GT86 2017-2020 true differences
 

What are the true differences under the hood mechanically? Even minor changes. For example; is the valvetrain, alternator, thermostat in a 2019(Post-Facelift) different or placed different from a 2017 (Post-Facelift)? I'm asking because I'm wondering if all aftermarket performance parts are interchangeable between 2017-2020. If they are I'm just going to but a 2020.

You can look at the Model Year update bulletins, but I don't recall there being anything significant between 2017 to 2020. Carplay/android auto stereos is one thing that comes to mind. Nothing appreciable that you would notice on a change out with normal bolt-ons. Only maybe stuff deep inside the engine or transmission