Nubank’s Flutter app reaches 125+ million customers across Brazil, Mexico, and Colombia — and its production architecture is engineered for exactly that scale. Backend Driven Content (BDC), Nubank’s in-house Server Driven UI framework, lets Clojure engineers write what the team calls “Lego manuals” — instructions, delivered over protobuf, that the Flutter client assembles into fully Nu Design System–compliant screens without an app store release. Alongside it, an App Experiments Platform measures every change in production, from frame timings to business KPIs. The system is powerful, and it’s the right answer at this scale. It’s also, by necessity, heavy: parsing overhead, network latency, and the gravitational pull of a large org make it hard for designers to probe what Flutter is actually capable of, or to know what “good” should feel like in the first place. So we built escape hatches. The most important one is the Concept Car — a separate, deliberately minimal Flutter app where Nubank’s design leads work in a tight loop with a small engineering team from LeanCode, our partner on the Nu Design System (NuDS). One repo. Figma open next to the IDE. No BDC in the path. It plays two roles: a performance benchmark that shows what pure Flutter can do at the high end, and a short-cycle collaboration space for the next generation of NuDS interactions, animations, and patterns — work that would be effectively impossible inside the production stack. In this joint talk, leadership from Nubank and LeanCode walk through the full picture: how BDC and the experiments platform enable safe, measurable shipping at scale; why the Concept Car exists alongside them rather than replacing them; and the local-build distribution tooling that gets prototypes onto a designer’s phone in minutes instead of hours. We’ll be honest about the tradeoffs — what the Concept Car is, what it isn’t, and what “agile” actually looks like inside an app this big.