1. You cannot automatically invest your HSA Fund. You have to be intentional about investing the fund somewhere. You can ask you HSA administrator to help you determine the right investment strategy for you.
2. There are different investment choices available for you, depending on who is managing your account. The most commonly options are investments in stocks, bonds, mutual funds and the like.
3. The availability of the fund is limited. You need to liquidate the money you used in your investment before you can use it. Fund transfer usually takes at least two days so you need to program anticipated expenses.