This study will look at how well CagriSema helps people with obesity lose weight compared to a "dummy medicine". CagriSema is a new medicine developed by Novo Nordisk. CagriSema cannot yet be prescribed by doctors. The study has two parts: First part is called the main phase and will last for 2 years, and second part is called the extension phase and will last for 1 year. In the main phase participants will either get CagriSema or "dummy medicine". Which treatment participants get is decided by chance and is not known by participants or the study doctor. In the extension phase participants will get either CagriSema or slowly reduce participants dose of CagriSema if participants had CagriSema in the main phase. Which treatment participants get is decided by chance and is not known by participants or the study doctor in both phases. If participants had "dummy medicine" in the main phase, participants will get CagriSema in the extension phase. Like all medicines, the study medicine may have side effects.