No. The advance premium tax credit (the subsidy) either from the Federal or the State of California are reconciled on the individual’s income tax return. If you made much more money than you estimated, then you would have to repay some or all of the subsidy you received during the year.
You can enroll in a health plan during Open Enrollment, which happens each Fall for the next calendar year. Outside the Open Enrollment period, you can only enroll in a health plan if you have a qualifying event such as getting married, losing group coverage, termination of MediCal, having a baby, etc. There is no “Open Enrollment” period for dental and vision plans which means you can purchase it anytime.