Can a person be eligible to purchase a ACA health plan in CA if they're on a B2 visa considering they have been in the USA for several years on prior work visas?

I know there are some recent changes with OBBBA that will kick in Jan 2027 but as per my understanding only affect PTCs not ACA eligibility.

Unfortunately, that's a benefits question, not an immigration one, so I'm not in a position to answer. That being said, I believe that your understanding is correct in CA.