Can I layer several brightening actives together, like alpha arbutin, tranexamic acid, niacinamide and azelaic acid?

These even-tone ingredients generally pair well and are commonly layered together, because they work through different, complementary routes: alpha arbutin and azelaic acid gently ease the pigment-making enzyme (tyrosinase), tranexamic acid quiets the UV-driven signal that tells skin to make more pigment, and niacinamide slows the handoff of pigment to surface cells. The main caution is not to overload reactive skin, since stacking many strong brightening actives at once can trigger irritation, so introduce one at a time, patch-test, and go slowly. Because any even-tone result fades without sun protection, daily sunscreen does most of the work. The concentration that matters is in the app. For persistent uneven tone, it is sensible to consult a doctor. This is cosmetic reference, not medical advice.