Well, they're three different things provided by four different institutions, and they track three different indices.
VTI is an ETF provided by Vanguard and tracks a total market index.
VTSAX is a mutual fund provided by Vanguard and tracks the same total market index.
SCHB is an ETF provided by Schwab and tracks a "Broad Market Index".
SPX is an index provided by S&P and tracks the S&P 500.
IVV is an ETF provided by Blackrock and tracks the S&P 500.
Whether ETF or mutual fund matters to you is one thing. You can research the differences if you like. Since I don't day trade and most of my money is tax deferred, I don't think it matters much for me.
Whether the company managing the product matters is another thing (and this is where expense ratio comes into play). Vanguard, Schwab, and Blackrock are all well respected generally, and the expense ratios of these are nearly identical.
The index it tracks probably matters the most if you ask me. But since all of the indices above include hundreds to thousands of companies and are all US-based and are all capitalization weighted, they are going to track each other very closely over time. I favor broader indices, which is why I personally invest in VTI and VTSAX.
You might learn about tracking error and how that relates to investment performance and expense ratios. Although unless you're talking about $10M or more, this is also a very minor issue in the big scheme of things.
You can also, FWIW, probably purchase any combination of ETF, index, and investment company you like. For example, if you like VTI because it is a Vanguard ETF but want to invest in the S&P 500 instead of the total market index, then you could buy VOO. If you want a mutual fund structure at Vanguard for the S&P 500, then you'd buy VFIAX. And so forth.