Which is false. Apple uses the same off the shelf DRAM chips from established manufacturers found in any PC/Phone/Laptop.

Unlike Flash storage, there is really not really bad performing DRAM per-se, just defective, And apple is no guarantee of free from defects with RAM fault in MACs being as common as in the rest of the PC industry.

The extra price is just the Apple tax.

That doesn't mean parts with some defects don't sneak into the final products but the rate is low enough that testing each chip you receive is not monetary feasible for consumer products so you just test the final assembled product to a degree, but even then, brand new Apple products are no strangers to having their mainboards replaced for a defective chip in the warranty period. Just ask Luis Rossmann or people working at the genius bar. That's why everyone recommends you get Apple care.

So the theory that Apple thoroughly tests each chip that goes in their products is false. It would be too expensive and eat into their fat margins.

What you do test, is samples from each batch you receive, to make sure your supplier did not pull a bait and switch and replaced your contracted parts with lower quality ones under the same part number and hope you would not notice but this is more common on low margin parts and Chinese suppliers are notorious for this since for them it's just regular business practice and see nothing wrong with it.

Source: my career as a hardware engineer.

We do BYOD on phones, so don’t have data there.

A friend of mine was once stationed in a jungle in South America. Most laptops couldn’t handle the humidity there. His IBM Thinkpad was the only one that held up.