Study to Compare Bictegravir/Lenacapavir Versus Current Therapy in People With HIV-1 Who Are Successfully Treated With a Complicated Regimen

Key Inclusion Criteria: * If plasma HIV-1 RNA measurements in the 6 months prior to screening are available, all levels must be \< 50 copies/mL. * At least one documented plasma HIV-1 RNA level measured between 6 and 12 months (± 2 months) prior to screening. This and any other HIV-1 RNA measurements documented in this period must be \< 50 copies/mL * Plasma HIV-1 RNA levels \< 50 copies/mL at screening. * Currently receiving a complex antiretroviral (ARV) regimen due to previous viral resistance, or intolerance, or contraindication to existing single-tablet regimens (STR), and on this regimen for at least 6 months prior to the screening visit. The criteria to define a complex regimen in this study are as follows: * A regimen containing a boosted protease inhibitor or a nonnucleos(t)ide reverse transcriptase inhibitor (NRTI) plus at least 1 other third agent (ie, an agent from a class other than NRTIs) (eg, bictegravir/emtricitabine/tenofovir alafenamide (coformulated; Biktarvy®)(BVY) + darunavir/cobicistat, BVY + etravirine), or * A regimen of ≥ 2 pills/day, or a regimen requiring dosing more than once daily, or * A regimen containing parenteral agent(s) (excluding a complete long-acting injectable regimen, such as intramuscular cabotegravir plus rilpivirine) as well as oral agents. * No documented or suspected resistance to bictegravir (BIC). * Estimated glomerular filtration rate ≥ 15 mL/min according to the Cockcroft-Gault formula for creatinine clearance (CLcr) who are not on renal replacement therapy. Key Exclusion Criteria: * Prior use of, or exposure to, lenacapavir (LEN) * Active tuberculosis infection * Chronic hepatitis B virus (HBV) infection Note: Other protocol defined Inclusion/Exclusion criteria may apply.