Do I have to use the HSA in conjunction with the HDHP?

Yes. IRS regulations stipulate that in order to be eligible for a Health Savings Account (HSA), you must be enrolled in a qualified High Deductible Health Plan. 

IRS regulations for 2021 require the HDHP to have a minimum  deductible of $1,400 for individuals and $2,800 for families. The premium for an HDHP generally costs less than a traditional health care plan, so the money that you save can be put in your HSA. You own and control the money in your HSA and decisions on how to spend the money are made by you.