I suddenly tripped across thematic investing in my etrade platform and the subject were interesting since most of them were in techfield: AI, gaming and cybersecurity. THere are several ETF I found and it is totally new for me. The composition HACK, GAMR and ESPO. I am interesting in purchasing few as they are composed of stock I am normally interested in. I also found article in which says "Jan 2019 saw 20B$ fly out of ETF market." Now this derives a question: I know ETFs are composed of various denomiation of stocks and allows the investor like small me to buy myriad of stocks composition at cheaper unit price of an ETF. Now the question is does ETF price solely follows the underlying stock composition or another factor in it? Let's say certain ETF has players: buyers and sellers and obviously it seem to have its own demand/supply independent of underlying security which is a composition of stock. What happens in case of where certain ETF A is considered and it consist of stocks B and majority or most of the underlying stocks B are performing well which should cause ETF to rise but most of the traders of that particular ETF a dumps it?