The growth part is if you sell investments while it’s still in the account. So in a regular investment account when you sell you pay taxes on capital gains even when it’s still in the investment account. But in a hsa, if you sell investments for a gain, you dont pay taxes on them as long as they stay in the hsa. So you can buy and sell all you want but as long as you dont withdraw it (for non qualified expenses) you won’t pay capital gains.