Study to Compare an Oral Weekly Islatravir/Lenacapavir Regimen With Standard of Care in Virologically Suppressed People With HIV-1

Key Inclusion Criteria: * HIV-1 RNA \< 50 copies/mL for ≥ 6 months before screening, as documented by: 1. One HIV-1 RNA \< 50 copies/mL immediately preceding the 24 weeks period prior to screening. 2. Within 24 weeks prior to screening, if HIV-1 RNA results are available, all levels must be \< 50 copies/mL. 3. During the 6 to 12 months period prior to screening, transient detectable viremia ≥ 50 copies/mL is acceptable ("blip") as long as it is not confirmed on 2 consecutive visits. * Plasma HIV-1 RNA levels \< 50 copies/mL at screening. * Are receiving guideline-recommended standard of care treatment such as International Antiviral Society (IAS), Department of Health and Human Services (DHHS), European AIDS Clinical Society (EACS) consisting of 2 or 3 ARVs for ≥ 6 months prior to screening and willing to continue until Day 1. Individuals in Treatment Group 2 must also be willing to continue their standard of care through at least Week 96. * Individuals assigned female at birth and of childbearing potential who engage in heterosexual intercourse must agree to use protocol-specified methods of contraception. Key Exclusion Criteria: * Prior virologic failure. * Prior use of, or exposure to, ISL or LEN. * Active, serious infections requiring parenteral therapy within 30 days before randomization. * Active tuberculosis infection. * Acute hepatitis within 30 days before randomization. * Hepatitis B virus (HBV) infection, as determined below at the screening visit: 1. positive HBV surface antigen OR 2. positive HBV core antibody and negative HBV surface antibody. Note: individuals found to be susceptible to HBV infection (eg negative hepatitis B surface antibody at the screening visit, regardless of prior HBV vaccination history) should be recommended to receive HBV vaccination. * Active hepatitis C virus (HCV) coinfection, defined as detectable HCV RNA. Note: individuals with prior/inactive HCV infection (defined as undetectable HCV RNA) may be enrolled. * Any of the following laboratory values at screening: 1. Creatinine clearance (CLcr) ≤ 30 mL/min according to the Cockcroft-Gault formula 2. Alanine aminotransferase (ALT) \> 5 x upper limit of normal (ULN) 3. Direct bilirubin \> 1.5 x ULN 4. Platelets \< 50,000/μL 5. Hemoglobin \< 8.0 g/dL Note: Other protocol defined Inclusion/Exclusion criteria may apply.