This is a multicenter Phase 3 clinical study, which aims to evaluate the effectiveness of an investigational drug called vimseltinib for the treatment of tenosynovial giant cell tumor (TGCT) in cases where surgical removal of the tumor is not an option. The study consists of two parts. In Part 1, eligible study participants will be assigned to receive either vimseltinib or matching placebo for 24 weeks. A number of assessments will be carried out during the course of the study, including physical examinations, blood tests, imaging studies, electrocardiograms, and questionnaires. MRI scans will be used to evaluate the response of the tumors to the treatment. Participants assigned to placebo in Part 1 will have the option to receive vimseltinib for Part 2. Part 2 is a long-term treatment phase in which all participants receive open-label vimseltinib.