This trial is for adults with advanced dedifferentiated liposarcoma who are not receiving other treatments. It compares a new drug, brigimadlin (BI 907828), which blocks a protein that helps cancer grow, with doxorubicin, an existing drug that damages cancer cell DNA. Participants receive either brigimadlin as tablets or doxorubicin through a vein. Doctors monitor tumor size and health regularly. The new drug aims to be effective and less toxic compared to conventional treatments.