Where is the main job market for information technology (business, finance) in Germany?

You are correct. Frankfurt is not only the largest financial centre in Germany, but in all of continental Europe and ranks among the world's leading financial centres. It is home to the European Central Bank, Deutsche Bundesbank, Frankfurt Stock Exchange and several large commercial banks. The European Central Bank is the central bank of the eurozone, consisting of 18 EU member states that have adopted the euro (€) as their common currency and sole legal tender. The Deutsche Bundesbank is the central bank of Germany and as such part of the European System of Central Banks. The Frankfurt Stock Exchange is one of the world's largest stock exchanges by market capitalization and accounts for over 90 percent of the turnover in the German market. In 2010, 63 national and 152 international banks had their registered offices in Frankfurt, including the headquarters of the major German banks, notably Deutsche Bank, Commerzbank, DZ Bank and KfW, as well as 41 representative offices of international banks.