How does having Medicaid as a secondary work? Does it pick up any leftover cost?

I can only seem to find conflicting information. I’m not on Medicaid yet but I’m trying to figure out my eligibility. I’m in pennsylvania if that’s relevant. So after something goes through my primary insurance, does Medicaid always cover everything left over if the doctor/facility accepts both my insurance and Medicaid? If not, how does it work? What if the only only take my primary? On the other hand, what if they only take Medicaid?

Also another question if anyone happens to know – I live with my parents, I’m 21, and disabled by my medical issues, so I have no income. They can’t afford to continue to pay my medical bills, but they make too much to get assistance if going by entire household income. As long as they don’t claim me as a dependent for 2022 (even though they technically could, as I’m eligible to be claimed), is it okay to only include myself in the household/say I’m my own household on the application? Is that the way to do it? Otherwise, I’m guessing the income of my parents would disqualify me.