Assuming you're not in Medicaid or Medicare:
Your employer pays your insurance premium
You may or may not have to top up that premium
Every individual member of the family has a maximum out of pocket: ie each member of the family has to hit (say) $2,000 of expenses before the insurance company will cover
Some drugs you have to pay a portion of the cost, called a co-pay. Some are free (eg pill, statins, preventative drugs)
Each doctor's visit has a co-pay unless it's an annual physical, or otherwise covered eg if a child has a specific issue covered by the policy
Some doctors and hospitals are in-network (ie benefit from your insurance policy coverage) some are out of network (you'll have to pay out of pocket). Not difficult to find suitable ones in network
Different policy for eyes
Different policy for dental
Basically it's really complicated, deliberately. They make mistakes, deliberately. The burden is on you. Doctors fear litigation more than patient fatalities so they go overboard. The industry hypes up prices massively because insurers are paying so insurers hike up premia massively
The whole system is backwards, corrupt, not the best for the patient always. BUT, some of the world's best facilities and research and doctors are here. It's functional, basically, but immoral imho