Adults between 18 and 80 years of age with ulcerative colitis can participate in this study. This is a study for people for whom previous treatment was not successful or who stopped previous treatment. The purpose of this study is to find out whether BI 3032950 helps people with ulcerative colitis. This study has 2 parts. In Part A, participants get BI 3032950 as an infusion into a vein every 4 weeks. After 12 weeks, doctors check whether the signs and symptoms of ulcerative colitis have improved. Before the results of this assessment are available, participants move on to Part B and get BI 3032950 as an injection under the skin. Participants whose results show clinical response after 12 weeks can continue treatment with BI 3032950. They get BI 3032950 injections under the skin every 4 weeks for up to 2 years. Participants visit their doctors every 4 weeks. During these visits, the doctors check the signs and symptoms of ulcerative colitis. This includes taking blood and stool samples. Doctors also do endoscopies. This is a procedure that uses a tube with a camera to look inside the body. The doctors also regularly check participants' health and take note of any unwanted effects.