On iPhone there’s actually a zero-install way—just use Apple’s built-in Find My. On the target device go to Settings → [Your Name] → Find My → Find My iPhone (make sure it’s on). Then jump into any browser, sign in at icloud.com and tap Find My to see its live location on a map. Apple’s approach is super reliable, private and baked right into iOS.
If the target is on Android, you can’t silently install anything—but if they already have Google Maps they can open Maps → Profile → Location sharing → “Share location” → get a link for a set time period, then send you that link. You’ll see their live pin in any browser. Android’s fine for this, but location-sharing setups vary by device and can chew through battery faster.