This study is about learning how a medicine called **Risankizumab** works in young people with **Ulcerative Colitis (UC)**. UC is a disease that makes the large intestine swollen and can cause bleeding. The study will find out if Risankizumab is safe and helpful for kids with moderate to severe UC. The study is for kids ages **2 to 18** and will have three different groups. The study has three parts: In **Part 1**, everyone gets the medicine by IV (through a vein) for **12 weeks**. In **Part 2**, kids get one of two doses by shots under the skin for **52 weeks**. In **Part 3**, kids get the medicine based on how they did in Part 2 for **208 weeks**. Kids will visit the hospital often for checkups, blood tests, and to see how the treatment is working.

• The study lasts for about **4 years** with regular hospital visits.
• The medicine is given in different ways: through a vein or by shots.
• Participants may face more treatment than standard care, like frequent checkups.