While steel seems to be the most common material to built the towers, taller ones seem to use a hybrid construction where the lower part is built of reinforced concrete and the upper one out of steel. The problem seems to be to transport large steel segments as German streets mostly don't fit anything with more than 4m diameter. Towers made out of only concrete seem to be pretty rare.

The brochure of one company: https://www.max-boegl.de/en/downloads-en/116-max-boegl-hybri...

(Disclaimer: I know nothing about this and just report back what I read on industry associations' and manufacturers' websites)