The goal of this clinical study is to learn more about the study drug, lenacapavir (LEN). The study will assess the safety, tolerability, and efficacy of long-acting LEN when combined with other medicines in adolescents and children living with HIV-1 who weigh at least 35 kg and have been treated before for HIV-1. The study will also see how easy it is for participants to take LEN as injection or an oral pill. The primary objectives are to evaluate the pharmacokinetics and safety of LEN in combination with optimized background regimen (OBR) in TE pediatric participants with HIV-1.