SPAXX or FCASH for investing in ETF

Hella confused. Just tryna etf and chill if you know what I mean. What is the best choice of the core. I’m in a high tax state (income tax) and I don’t care about how accessible my money is (it can marinate). Literally don’t know much about all this, just trying to be the guy that invests at 20 rather the guy that invests at 30 if u know what I mean😉. Help me:/