The ETFs are generally rebalanced once a quarter, so yeah, it really is a big deal when a company enters one of the main indices, and the price starts to move u

The ETFs are generally rebalanced once a quarter, so yeah, it really is a big deal when a company enters one of the main indices, and the price starts to move up in anticipation of the listing, then stays up because the ETFs, as you intuit, have to start buying it as a component of the market. Supermicro entering S&P500 and Arm entering Nasdaq 100 being good examples recently.