Clarification on health plans for my 19yo step child

I have an adult stepson (biologically female) who has some pretty extensive health challenges.

His (19m) bio father is required to carry health insurance for both of my step kids. We found out bio dad’s insurance doesn’t cover anything towards durable medical equipment. I assume he has a high deductible low premium plan.

My health insurance is an hmo with fantastic coverage. Open enrollment is around the corner and my wife and I are considering adding her kids to our plan primarily for the one adult kid mentioned above.

My bio kids are under a plan their mom pays for, so we currently have a “Employee + Spouse” plan only for my wife and I.

I spoke to my insurance company and they explained that if we add the kids (step kids only), their dad’s plan pays first, and then ours would kick in. The rep on the phone couldn’t with confidence answer some of my questions.

I have 2 questions for my Reddit friends:

1- does this mean their dad’s insurance deductible amount for 19yo must be met before my plan will kick in? If not, ELI5 how it’s split between the two plans.

2: since 19yo is a legal adult, is he still obligated to be on bio dad’s plan? Is there a workaround to make my plan primary?

Any advice is appreciated. Thanks