My HSA has multiple options on where to park the money. But the default option is just the savings account with .2% interest. If I want to move the money into any sort of investment fund, I have to make a point of doing that. Your HSA may have a similar set up.
Having said that, you need to have a plan for what you want to do with your HSA money.
Are you planning to spend it on medical expenses as they come up over time? In that case, you probably don't want your funds in the stock market. Too volatile for funds you may spend at any time now.
On the other hand, some people plan to use their HSA as a back door IRA, never touching the money until they're 65. Any medical expenses that come up will be covered with other funds. Those people would be wise to put their the money inside their HSA into a stock fund.
Either way, it's cool to get the tax break.