Study of ALXN1920 in Adult Participants With Primary Membranous Nephropathy (PMN)

Inclusion Criteria: * Participants who have a documented diagnosis of PMN, established by positive antiPLA2R antibody level (\> 50 RU/mL) at Screening, which must be confirmed by a central laboratory * Participants are willing to receive the background Standard of Care (SoC) * Participants at high risk for disease progression, defined as: 1. Receiving ACE inhibitor or ARB for a minimum of 8 weeks prior to Screening, with the dose titrated to the maximally tolerated level. Participants with less than 8 weeks on ACE inhibitor or ARB before Screening or who have not yet reached maximally tolerated dose will enter the Run-in Period. 2. Participants who are on ACE inhibitor or ARB for a minimum of 8 weeks with Systolic Blood Pressure \< 140 mmHg in ≥ 75% of the readings within last 8 weeks. 3. Having two proteinuria measurements with each \> 3.5 g/day, the second measurement showing ≤50% decrease from the first measurement. * All participants must receive prophylactic treatment with appropriate antibiotics while receiving Rituximab (RTX), and be willing to be vaccinated against Neisseria meningitidis Exclusion Criteria: * Estimated glomerular filtration rate (GFR) \< 60 mL/min/1.73 m\^2 during Screening * Documented rapid deterioration of kidney function * History of life-threatening Nephrotic Syndrome within 1 year before Screening * Diagnosis of anti- phospholipase A2 receptor (PLA2R) negative membranous nephropathy (MN) or anti-PLA2R positive MN but Screening serum anti-PLA2R \< 50 RU/mL or kidney disease other than PMN * History of kidney transplant or planned kidney transplant or dialysis during the Treatment Period * History of other solid organ (heart, lung, small bowel, pancreas, or liver) or bone marrow transplant; or planned transplant during the Treatment Period * History or presence of any clinically relevant co-morbidities * History of intolerance or hypersensitivity to ACEi or ARB * Use of SGLT2i, MRA, or ERA within 8 weeks prior to randomization and throughout the study period Note: Additional inclusion/exclusion criteria may apply, per protocol.