RE:central bank of japan now owns 75% of etf'sBecause Japan is engaged in QE, and have to buy an insane amount of bonds and other securities in order to support the program. They probably slowly ran out of bonds (public & private) to buy & started purchasing etfs to prop up the market.
How monetary policy works, open market operations. Buy bonds & securities from public and give cash to maintain near 0 interest rates.