Where to Invest?

Basic info: no debt, house paid off. Married. 50s. Low monthly expenses. Current income ~150. Have $500k in MMA. Want to invest properly. Very, very minimal understanding of investments. Very risk averse. I’ve researched for months and feel barely more knowledgeable than when I started, but I’m proposing opening a ROTH and HSA with Fidelity and plonking the max allowance in those. Question: do I need to select how the $ is then invested once deposited? And, if so …. Help! No idea on that. After that, again with poor understanding of things, it seems like a decent amount in an S&P 500, perhaps? And same as above - do I need to allocate/select where that $ is actually invested or is that automatic? Any help/advice appreciated - please don’t use (or if you do, explain) too many acronyms as I get lost and don’t understand them. Thank you!