This is one of the weird provisions. The ACA doesn't require the young adult to also be a dependent for tax filing purposes, to be covered by the health plan. So, I looked for some easy ways to explain it and found these:

https://www.kitces.com/blog/health-savings-accounts-hsa-dependents-children-age-26-hdhp-taxes-contri...

"the IRS imposes certain requirements for who can contribute to an HSA: The individual must be covered by a High Deductible Health Plan (HDHP) (and have no other health coverage or be enrolled in Medicare) and they may not be claimed as a dependent on someone else’s tax return. Notably, the account owner does not have to be covered under their own healthcare plan, so a young adult who is covered under their parents’ HDHP plan (and who cannot be considered a dependent on their parents’ tax return) would potentially be eligible to contribute to their own HSA. Further, while spouses can only make combined contributions up to the family maximum contribution limit ($7,300 in 2022), non-spouses covered under the same health plan can make contributions to their own HSA up to the family limit as well!"

https://americanfidelity.com/blog/reimburse/hsa-mistakes-dependents/

"Do you have a child who is covered on your qualified HDHP who is not a tax dependent? If yes, you cannot use your HSA to cover his or her out-of-pocket medical expenses. The child will need to open his or her own HSA to cover out-of-pocket medical expenses."

"Because the employee’s HSA funds can’t be used for this dependent, the adult child may wish to establish a separate HSA for his expenses. However, there is something unique about this situation that many don’t realize. Because this child is covered on a family-qualified HDHP, opening a separate HSA would allow the child to contribute up to the allowed maximum family contribution of $7,300 (this maximum increases $7,750 starting in 2023). So, the parent (your employee) could have an HSA and contribute the allowed maximum family contribution of $7,300 and the dependent adult child could contribute up to $7,300.

This allows the employee’s HSA funds to be used for the spouse and other qualified dependents, while the adult child has his own funds to use for eligible medical expenses.

Keep in mind that the primary account holder (your employee) cannot use pre-tax salary reductions to contribute to the adult child’s HSA. Also, the child’s HSA can only be used for the child’s eligible out-of-pocket medical expenses."