Study of Bictegravir/Lenacapavir in Children and Adolescents With HIV-1

Key Inclusion Criteria: * Age and body weight at screening: * Cohort 1: ≥ 12 years to \< 18 years weighing ≥ 35 kg. * Cohort 2: ≥ 6 years to \< 12 years weighing ≥ 25 kg to \< 35 kg. * Cohort 3: ≥ 2 years to \< 6 years weighing ≥ 10 kg to \< 25 kg. * On a complex ARV regimen. Complex regimens are any ARV therapy that is not a single-tablet regimen taken once daily (eg, \> 1 tablet or any other formulation a day). * Documented plasma HIV-1 ribonucleic acid (RNA) levels must be \< 50 copies/mL (or undetectable HIV-1 RNA level according to the local assay being used if the limit of detection is \< 50 copies/mL) in the last 6 months prior to screening (at least 1 measure prior to screening). * Plasma HIV-1 RNA levels \< 50 copies/mL at screening. * No documented or suspected resistance to integrase strand transfer inhibitors (mutations T66A/I/K, E92G/Q/V, G118R, F121C/Y, G140R, Y143C/H/R, S147G, Q148H/K/R, N155H/S, or R263K in the integrase gene). * The following laboratory parameters at screening: * Estimated glomerular filtration rate ≥ 30 mL/min/1.73 m2 using the Bedside Schwartz formula. * Absolute neutrophil count \> 0.50 cells/L (\> 500 cells/mm3). * Hemoglobin ≥ 85 g/L (\> 8.5 g/dL). * Platelets ≥ 50 cells/L (≥ 50,000 cells/mm3). * Hepatic transaminases (aspartate aminotransferase and alanine aminotransferase) ≤ 5 x upper limit of normal. * Total bilirubin ≤ 23 μmol/L (≤ 1.5 mg/dL) and direct bilirubin ≤ 7 μmol/L (≤ 0.4 mg/dL). Key Exclusion Criteria: * CD4 cell count \< 200 cells/mm\^3. * CD4 percentage \< 20%. * Life expectancy ≤ 1 year. * An opportunistic illness indicative of Stage 3 HIV diagnosed within the 30 days prior to screening. * Evidence of active pulmonary or extrapulmonary tuberculosis within 3 months prior to screening. * Acute hepatitis within 30 days prior to screening. * Positive hepatitis C virus (HCV) antibody with detectable HCV RNA (participants positive for HCV antibody will have an HCV RNA test performed). * Positive hepatitis B surface antigen (HBsAg) or positive hepatitis B virus (HBV) core antibody (antibody against hepatitis B core antigen \[anti-HBc\]) at screening. If a participant is negative for HBsAg and positive for anti-HBc but HBV DNA is undetectable, the participant may be enrolled. * A history of or current decompensated liver cirrhosis (eg, ascites, encephalopathy, or variceal bleeding).Current alcohol or substance use judged by the investigator to potentially interfere with the participant's study compliance. Note: Other protocol defined Inclusion/Exclusion criteria may apply.