Back in the olden days, if you wanted to buy the S&P, you had to have a lot of money so you can buy the shares. Then somebody had the bright idea of making a fund that just buys the S&P, and then sells small pieces of it to investor without huge mountains of capital. Enter the ETFs.
The guy running the ETF, of course, doesn't do it for free. He skims a little bit of money off the top. This is the "fee". The major S&P ETFs all have tiny fees, in the percents of a percent. If you're buying the index, you're probably looking at gains (or losses) to the tune of 5, 10, 20% - unless you're doing something really silly, you wouldn't even notice the fee.
As often happens, when one guy starts doing something and making money, there will immediately be copycats. So now we have competing ETFs all providing the same service. You are technically a competitor as well, since you could compete with all these funds by just buying a basket of shares yourself, thereby running your own private fund for yourself.
The reason this stuff even started was that people said, "well why bother with mutual funds when they charge such huge fees and still don't beat the index anyway", so the index ETFs are supposed to be a low cost alternative to mutual funds. Thus one thing ETFs compete on is fees: You can see how
VOO has lower fees than
IVV, in keeping with Vanguard's philosophy of minimal management (and management fees). Incidentally, if you buy the shares directly, you wouldn't charge yourself fees, but you would have to pay commissions on each stock and it would destroy you - another benefit of the ETFs.
Moreover, these ETFs claim they track the index, but of course there is no real way to peg an asset to another. So they ensure tracking by keeping a carefully curated portfolio. Of course nobody is perfect, and there's tracking error. You can in theory compare the ETFs in this respect and buy the one with the least tracking error. However they all basically track very closely, again the error is fractions of the percent, if it is a legitimate concern in your books then you're not doing index investing right. The actual prices of each fund may vary, but the price hardly matters - the key metric is does it go up 20% when the index goes up 20%? And they all do.
So what do you compare them on? Well, typically companies offer people perks to attract them to their own product. If you are a Fidelity customer, and you buy
IVV, they will waive your commission if you hold it for a month. I believe Vanguard will also sell
VOO for free. But for instance Fidelity will take commission from
VOO trades and vice versa. So, this would be your main factor. Though, then again, you can just make an account on Robinhood and they're all commission free.
A second factor is reliability of the operator. Frankly, I doubt any of these operators are at all untrustworthy, and you'd be buying your own broker's ETF anyway, and presumably you already went with the most trustworthy broker.
Besides that, like I said, there's trivial matters like fees and tracking error, but you might as well just flip a coin. It doesn't really matter.