To present a sheet in SwiftUI, you use the .sheet() modifier. This modifier takes a binding to a Boolean value that controls whether the sheet is presented, and a closure that returns the content view for the sheet. For example:

struct ContentView: View {
    @State private var showingSheet = false

    var body: some View {
        Button("Show Sheet") {
            showingSheet = true
        }
        .sheet(isPresented: $showingSheet) {
            SheetContentView()
        }
    }
}

struct SheetContentView: View {
    @Environment(\.dismiss) var dismiss

    var body: some View {
        VStack {
            Text("This is a sheet!")
            Button("Dismiss") {
                dismiss()
            }
        }
    }
}

The @Environment(\.dismiss) property wrapper is used within the sheet's content to programmatically dismiss it.

The .alert() modifier is used to present alerts. It can be configured with a title, a message, and a list of Alert.Button instances. Similar to sheets, a binding controls its presentation.

struct ContentView: View {
    @State private var showingAlert = false

    var body: some View {
        Button("Show Alert") {
            showingAlert = true
        }
        .alert("Important Message", isPresented: $showingAlert) {
            Button("OK") { /* Action */ }
            Button("Cancel", role: .cancel) { /* Action */ }
        } message: {
            Text("This is a simple alert message.")
        }
    }
}

Alerts can have different button roles (e.g., .destructive, .cancel) and can be customized to include text fields for user input using .alert(isPresented:withStyle:actions:message:).