How to make width of view equal to superview in SwiftUI

You need to use .frame(minWidth: 0, maxWidth: .infinity) modifier

Add the next code

        Button(action: tap) {
            Text("Button")
                .frame(minWidth: 0, maxWidth: .infinity)
                .background(Color.red)
        }
        .padding(.horizontal, 20)

Padding modifiers will allow you to have some space from the edge.

Keep in mind that the order of modifiers is essential. Because modifiers are functions that are wrapping the view below (they do not change properties of views)