fwiw, you can configure a 13" Macbook Pro with an Intel chip up to 32GB. But I agree, I wish they launched the new M1 based 13" Pros with up to 32GB RAM
The low end Air only has 7 GPU cores compared with 8 on the one with more storage. So they must be disabling a bad core and selling it the cheap model. Other than that all these machines use the exact same CPUs. Which means that an iMac or 16" MBP are probably going to use a M1X or something with more cores.
Yes. Chip fabrication is super sensitive to the condition of the silicon wafer used. Chip companies talk about yields, because some percentage of chips can't, for example, be run at the highest clock rate. Indeed, some can't run reliably at all. If there is a microscopic flaw on the wafer that ends up being where one of the cores is located, disabling that core altogether is an option to keep that silicon marketable.