This study will look at how well CagriSema and cagrilintide help children and adolescents with excess body weight lose weight. The study has 2 parts: main and extension study. In the main study, participants will either get CagriSema (a new study drug), cagrilintide (a new study drug), semaglutide (a drug that doctors can already prescribe to adolescents and adults) or placebo (a placebo looks like the treatment being tested, but doesn't have any active ingredients in it). Which treatment participants will get is decided by chance. Participants who get semaglutide in the main study will not take part in the extension study. If participants take part in the extension study, they will get either CagriSema or cagrilintide in this part of the study. Like all drugs, the study drugs may have side effects. The total time participants will be in the main study is about 1 year and 6 months. If participants take part in the extension study, the total time is about 4 years and 10 months.