Manually Opening SideBar in SplitView as Popover

You can programmatically toggle the columnVisibility by creating a State value of type NavigationSplitViewVisibility.

For example

struct ContentView: View {
    @State private var columnVisibility: NavigationSplitViewVisibility = .detailOnly

    var body: some View {
        NavigationSplitView(columnVisibility: $columnVisibility) {
            Text("Hello world")
        } detail: {
            Button("Toggle columnVisibility") {
                toggleSidebar()
            }
        }
    }
    
    private func toggleSidebar() {
        if columnVisibility == .all {
            columnVisibility = .detailOnly
        } else {
            columnVisibility = .all
        }
    }
}