You determine how much to contribute to an HSA each year, and can change mid-year if you want, up to the limit on HSA contributions. Your employer may choose to make a matching contribution of some amount.

When you have an HSA-eligible expense, you can pay directly with HSA funds or with after-tax dollars. If you pay with after-tax dollars you can either do nothing else or reimburse yourself from the HSA (if you have sufficient funds available in the HSA).

If you reimburse yourself, the money you take out of the HSA is not replaced by anyone.

There is no time limit to reimburse yourself from the HSA, but if you delay for years make sure you keep really good records.