Fund management companies' (e.g. Vanguard, BlackRock iShares, etc.) annual reports include a complete list of securities held by each ETF. This complete list is only published in annual or semi-annual reports, so the data is likely outdated as soon as the report is published. I noticed that there are several third-party websites that list the holdings of ETFs. How do the websites know each ETF's holdings at any given time? Do they have some kind of special arrangement with the fund management companies to obtain the data?
The premise of the question does not appear to be true. Here's an iShares webpage for the EEM ETF that shows the complete list of holdings as of the previous trading day: