Researchers are looking for ways to treat a type of liver disease caused by elevated liver fat, called metabolic dysfunction-associated steatohepatitis (MASH). MASH was formerly called non-alcoholic steatohepatitis (NASH). Researchers want to learn if a study medicine called efinopegdutide can treat MASH.The goals of this study are to learn: * If efinopegdutide can lower the amount of fat, inflammation, and scarring (fibrosis) in the liver * About the safety of efinopegdutide and how well people tolerate it