Is there a pharmacy that requires the use of an insurance company or a PBM? I thought involving them was always voluntary (and someones people would pay 100% of the “cash” price because it can be less than a copay).

Yes for PBM because that is where the pharmacy buys the drug, not the manufacturer. Manufacturers usually can't sell direct to pharmacies.

You can usually pay cash at a pharmacy, skipping insurance

The pharmacy will buy the drug from a distributor, not the PBM. The PBM manages the insurance.

These may all be standalone operations, or one corp with its hands in each pocket: PBM, pharmacy, insurer and distributor.

That is correct. My point was to highlight that there is a lot more to the supply chain than manufacturer to pharmacy, with intense negotiations and fake pricing at each step