The investment account is not interest-bearing but you may receive dividends from ETFs. Dividends will automatically be swept back into your HSA cash account where you can decide if you want to move them back into your investments.
Your statement may not include the dividends if they are issued after the statement generation date. The statement includes activities as of the end of the month, but you may receive the dividends after that period.
The cash account is interest-bearing and does not require any action from you.