I am an old backend engineer with 20+ years of experience, and I am possibly missing something here, but could any FAANG engineer around ELI5 why you need so ma

I am an old backend engineer with 20+ years of experience, and I am possibly missing something here, but could any FAANG engineer around ELI5 why you need so many engineers ? Or in purely quantitative terms: What percentage of the whole work force is devoted to the apps (IOS, Android) ? Thanks.