The goal of this clinical study is to learn about the safety and tolerability of bictegravir/lenacapavir (BIC/LEN) and to learn how the study drug interacts with the body in virologically suppressed (VS) children and adolescents with human immunodeficiency virus type 1 (HIV-1) on a stable and complex antiretroviral (ARV) regimen. The study will also assess the safe loading dose of LEN and pharmacokinetics (PK) of BIC/LEN. The primary objectives of this study are: * To evaluate the steady-state PK of BIC and LEN and confirm the dose of the LEN loading dose and BIC/LEN FDC in VS children and adolescents with HIV-1. * To evaluate the safety and tolerability of BIC/LEN through Week 24 in VS children and adolescents with HIV-1.