As of market close on 6/17, a simple market cap weighted version of $ETF would look something like this:

$AMZN: $1.32t
+ $MSFT: $1.49t
+ $AAPL: $1.53t
= $4.43t total

$AMZN: 1.32/4.43 = .30
$MSFT: 1.49/4.43 = .33
$AAPL: 1.53/4.43 = .35

This is how funds like $SPY and $QQQ work.