If an expense ratio of an ETF is 0.15% and the ETF is trading at 100, then when I buy the ETF, do I pay 100? Or do I pay 100.0015?
The expense ratio reduces the return of the ETF; your scenario of paying 100.0015 is that of a load. Most (all?) ETFs can be bought without paying a load (sales charge as a percent of amount invested), and some ETFs can be bought without paying a brokerage fee (fixed or variable charge for a buy transaction just like buying any other stock through the brokerage) because the brokerage has waived it. Your broker might charge fees for both buying and selling shares in an ETF, but in any case, this is quite separate from the expense ratio.