SwiftUI List inside ScrollVIew

It is possible but not when the List is using the full screen.

In the code example I used GeometryReader to make the list as big as possible. But you can also remove the GeometryReader and just insert a fixed dimension into .frame()

struct ContentView: View {
    
    @State private var numbers = [1,2,3,4,5,6,7,8,9]
    
    var body: some View {
        GeometryReader { g in
            ScrollView {
                Text("header")
                List {
                    ForEach(self.numbers, id: \.self) {
                        Text("\($0)")
                    }
                    .onDelete { index in
                        // delete item
                    }
                }.frame(width: g.size.width - 5, height: g.size.height - 50, alignment: .center)
            }
        }
    }
}

There is no need for two scrolling objects. You can also use section for this:

@State private var numbers = [1,2,3,4,5,6,7,8,9]

var body: some View {
    List {
        Section.init {
            Text("Header")
        }
        ForEach(numbers, id: \.self) {
            Text("\($0)")
        }
        .onDelete { index in
            // delete item
        }
    }
}