Currency exchange

Hello guys,
on my page https://hausplaner-24.com

1. where to put in / set up currency exchange rates? Got the currency dropdown on, but it will allways return to Euro.

2. why, when “add tax” is set “yes” and Tax=19%, why doesnt it add 19% to the price of meta_field “price”, to be seen in frontend?

3. even when I have the “Flat Rate (percentage)” on 19 % in Main-Menue=>Tax&Shipping.. there is nothing added. Why?

Thank you, Daniela