Brightline-3: A Study to Find Out Whether Brigimadlin in Combination With Ezabenlimab Helps People With Advanced Soft Tissue Sarcoma

This study is open to adults with specific types of advanced soft tissue sarcoma. People with undifferentiated pleomorphic sarcoma (UPS) or myxofibrosarcoma (MFS) can join the study if they have a normal version of the TP53 gene. This is a study for people whose earlier treatment isn't working anymore, and their doctors suggest a new treatment to stop the sarcoma from getting worse. The purpose of this study is to compare a medicine called brigimadlin in combination with another medicine called ezabenlimab with chemotherapy. Brigimadlin is a so-called MDM2-p53 antagonist that is being developed to treat cancer. Ezabenlimab is an antibody that may help the immune system fight cancer. Participants are put into 3 groups by chance: * Ezabenlimab group: Participants receive ezabenlimab as an infusion into a vein every 3 weeks * Brigimadlin + ezabenlimab group: Participants take brigimadlin as tablets and receive ezabenlimab as an infusion into a vein every 3 weeks * Chemotherapy group: Participants get chemotherapy as an infusion into a vein on 2 days every 3 weeks. Chemotherapy is a combination of gemcitabine and docetaxel which is often used in the treatment of sarcoma. There are twice as many participants in the brigimadlin + ezabenlimab group and in the chemotherapy group, compared to those in the ezabenlimab group. Participants can continue treatment in the study as long as they benefit from it and can tolerate it. Doctors regularly check the size of the tumor and check whether it has spread to other parts of the body. The doctors also regularly check participants' health and take note of any unwanted effects. Participants in this study use an app on a mobile phone to regularly answer questions about their health and well-being. This is to find out if their quality of life is changing.