This is a study to evaluate the efficacy and safety of AZD0780 in adults with HeFH and elevated LDL-C, either with clinical ASCVD and LDL-C levels of 55 mg/dL or higher or without clinical ASCVD and LDL-C levels of 70 mg/dL or higher. AZD0780 is a small molecule that reduces the amount of LDL-C in the blood. Placebo will be used for comparison, and neither the participants nor the Investigators will know who is receiving the AZD0780 medication and who is receiving the placebo until the end of study. The total length of the study for an individual participant will be up to approximately 56 weeks, including a screening period of up to 14 days, treatment with AZD0780 or placebo for 52 weeks, and a safety follow-up period of 10 days.