The way I understand it.
VTI is every stock in america bundled into one stock.
VOO is the top 500 stocks in america bundled into one stock.
The stock bundles above are called ETFs (exchange traded funds), and you can buy/sell them like a regular stock in the stock market from a brokerage. E.g. APPL and VTI are bought and sold the same way.
VTSMX is like VTI because it trades every stock in america, but it cannot be traded like a regular stock. VTSMX isn’t an ETF it’s a Mutual Fund. Mutual Funds are basically many investors pooling their money together and buying securities. Your equity (how much you own of the mutual fund) is based on how much you put in.
So all these investment vehicles do similar things but give you flexibility in how to do them. So, you might rightfully ask, “what are the advantages of each?” Well ETFs are easier to trade because they are just like stocks, and Mutual funds are easier to invest into because there is no stock price (vti is like 144 or something) with mutual funds you can simply invest a set number without worrying about stock price. The investments with every stock in america have higher highs and lower lows than the top 500 stocks because there are more variables.
But no matter what you choose as your investment, what matters is that investing is on your mind and you’re picking investments that long term will make you come out on top. Good luck friend.
TLDR: VTI is every stock in america bundled into one stock. VOO is the top 500 stocks in america bundled into one stock. VTSMX is basically VTI but the purchasing and trading is handled slightly differently.