> would you say that e.g. an ETF tracking MSCI ex-US is not a passive fund?

Assets aren’t passive or not; investing styles are. The degree to which one’s returns earn from, or are expected to earn from, selection effects determines if you’re investing passively or actively. If I say trade SPYs, I’m an active investor. If I buy and hold a custom broad-market benchmark, I’m passive. Buying MSCI ex-US without a hedge is an active investment decision. If it’s bought and held it’s more like a passive strategy over time, provided the U.S. doesn’t dramatically over or underperform the global markets.