Here's a solution. Not the most elegant, but it works.

JSON structure:

[
    {
        "brand": "BMW",
        "models": [
            {
                "name": "335"
            },
            {
                "name": "x5"
            }
        ]
    },
    {
        "brand": "Tesla",
        "models": [
            {
                "name": "CyberTruck"
            },
            {
                "name": "X"
            },
            {
                "name": "S"
            }
        ]
    },
    {
        "brand": "Porsche",
        "models": [
            {
                "name": "911"
            },
            {
                "name": "Taycan"
            }
        ]
    }
]

Models:

struct Manufacturer: Decodable {
    let brand: String
    let models: [Model]
}

struct Model: Decodable {
    let name: String
}

And the actual view and its model:

struct ContentView: View {

    @ObservedObject private var model = ContentViewModel()

    var body: some View {
        VStack {
            if model.selectedManufacturer == -1 {
                Text("No Brand Selected")
            } else {
                Text("\(model.carData[model.selectedManufacturer].brand)")
            }

            Picker(selection: $model.selectedManufacturer, label: Text("Brand")) {
                Text("None")
                    .tag(-1)

                ForEach(0 ..< model.carData.count) { carIndex in
                    Text(self.model.carData[carIndex].brand)
                        .tag(carIndex)
                }
            }

            if model.selectedManufacturer != -1 {
                Picker(selection: $model.selectedModel, label: Text("Model")) {
                    Text("None")
                        .tag(-1)

                    ForEach(0 ..< model.models.count, id: \.self) { modelIndex in
                        Text(self.model.models[modelIndex].name)
                            .tag(modelIndex)
                    }
                }
            }
        }
    }
}

class ContentViewModel: ObservableObject {
    let carData: [Manufacturer]

    @Published var selectedManufacturer = -1 {
        didSet {
            // reset the currently selected model to "None" when the manufacturer changes
            selectedModel = -1
        }
    }
    @Published var selectedModel = -1
    var models: [Model] {
        if (0 ..< carData.count).contains(selectedManufacturer) {
            return carData[selectedManufacturer].models
        }

        return []
    }

    init() {
        let url = Bundle.main.url(forResource: "TestCars", withExtension: "json")!
        let data = try! Data(contentsOf: url)
        carData = try! JSONDecoder().decode([Manufacturer].self, from: data)
    }
}