Can someone please help me understand these plans?

I’m starting a new job and have three plans to choose from. All from Anthem Blue Cross Blue Shield. I understand the deductibles, max out of pocket, etc. (I think). I’m mainly trying to understand the copay/how much I’ll be charged if I go to a normal checkup or something of the sorts.

The format is: deductible/coinsurance/Max out of pocket

Anthem Silver PPO 4000/20%/8550 This shows a primary care visit for an injury, etc. is $40 a visit and 20% coinsurance. Preventive care no charge

It’s the most expensive

2. Anthem Silver PPO 4000/20%/7000 w/HSA This shows a primary care visit has a 20% coinsurance w/o a dollar amount (main thing I’m confused about) No charge for preventative care

Middle price

3. Anthem Silver PPO 6850/0%/6850 This shows a primary care charge has 0% coinsurance, w/o a dollar amount No charge for preventative

Cheapest price

So why is the cheapest plan the one where I pay 0% coinsurance? If I rarely go to the hospital, but don’t want to be surprised with an outrageous bill if something does come up, what would you suggest?

I’ve called anthem and they said they can’t help. I’ve asked HR a couple questions but feel dumb asking these simple questions at a new job.