@pcjr_ea All the components you listed are soldered onto the logic board so when one fails Apple just replace the whole unit. A bad memory chip may not have been heat damaged, in fact it's unlikely that a RAM chip would be, but they would have just replaced the whole board. This is a great teardown of the machine showing all parts -
https://www.ifixit.com/Teardown/MacBook+Pro+16-Inch+2019+Teardown/128106The aluminium casing is designed to help dissipate heat, and it does, provided the machine is on a hard surface with air able to circulate underneath. If you're gaming then a cooling fan or raised surface of some kind is an absolute must. That doesn't just apply to MBPs but any laptops, Mac or Windows.
It's actually very rare that a Mac will overheat to the point that components would start failing. Of course, if you play on a bed with your Mac resting on the duvet then yes, heat is going to be a huge factor and most likely shorten the life of your Mac. But the OS is designed to throttle power to the hardware to prevent overheating. You know when people report that their Mac is getting hot and switching itself off? It's shutting itself down to prevent damage. And most of the posts I've ever answered from people who this is happening to is down to them playing on a nice soft surface.
Absolutely yes to Applecare, I have always bought it on my Macs. It's a small price to pay when you're paying £3k+ for a computer. And Apple's customer service has always been second to none.