The Home Buyers' Plan is a loan from your own RRSP, repaid over 15 years starting the second year after withdrawal, so it reduces retirement savings temporarily rather than costing anything upfront. The FHSA works differently: contributions are tax deductible and the eventual withdrawal toward a home is tax free, which makes it closer to a grant in effect than a loan.
Using both at once, and timing the withdrawals to line up with a mortgage pre-approval and possession date, is where most of the value gets left on the table if it is done wrong. A buyer who withdraws too early or in the wrong tax year can lose part of the benefit.
Joel Dyck coordinates that timing with your mortgage broker so the accounts and the purchase timeline actually match up. Get your hand-reviewed valuation from Joel Dyck.