ETFs (Exchange Traded Funds) are "amalgams" of stocks. (That what funds are.)
Therefore, if you sell shares of an ETF, you're effectively selling the stocks which belong to the ETF.
SPY is an ETF containing all 500 stocks in the S&P 500 Index. Thus, if loads of people who own
SPY suddenly decide to sell their shares, the individual stocks would also be sold. That would put downward pressure on the stocks which make up the S&P500.
AAPL is part of the S&P, and so would also fall.
What you didn't mention is that there are many other funds which mirror the S&P500. Examples are:
Some of those funds are Really, Ginormously Huge. Since you only mentioned
SPY and not any of the others, I'm dubious as to how much impact there would be.
There would be some panic selling by others ("What do all these other sellers know that I don't know? Better sell!!"), but there would also be some bargain buying by others ("The fundamentals are good, so let me scoop up more at a bargain.)