Change background SwiftUI

I am not sure I understand what you want, but…

To get a blue background on the pickerView (the roundedRect), I modified as follows :

Code Block 
        }.padding()
        .background(Color.blue)		// <<<---- ADDED
        .clipShape(RoundedRectangle(cornerRadius: 16, style: .continuous))
        .overlay(
            RoundedRectangle(cornerRadius: 16)
                .stroke(Color.gray, lineWidth: 3)
        )

What I want is (for example) when I tap on the Text(arr[0]) at line 12 the background becomes grey and if I tap on Text(arr[1]), Text(arr[0]) returns to be normal background and Text(arr[1]) becomes grey

This is just a quick hint.

Create a State didTap (here it changes the color of the whole picker, which may not be what you want ultimately, but that gives an idea I hope).
In fact, you should get a state var for each item in the picker.
But the problem is that you just have a Text, not a full frame around the text that you would change color.
May be you have to adapt your code.

Code Block 
struct PickerView: View {
    
    var arr: [String] = ["Easy", "Medium", "Hard"]
    var h: CGFloat = 50
    var w: CGFloat = 320
    
    @State private var didTap:Bool = false
    var body: some View{
        HStack(spacing: 40){
            ForEach(0..<arr.count){ i in
                HStack(spacing: 25){
                    Text(arr[i])
                        .bold()
                        .onTapGesture {
                            self.didTap.toggle()
                            switch i {
                            case i: input.indi = i
                            default: return
                            }
                            print(i)
                        }
                    if(i < arr.count - 1){
                    Divider()
                        .frame(height: 25)
                    }
                }
            }
        }.padding()
        .background(didTap ? Color.blue : Color.yellow)
        .clipShape(RoundedRectangle(cornerRadius: 16, style: .continuous))
        .overlay(
            RoundedRectangle(cornerRadius: 16)
                .stroke(Color.gray, lineWidth: 3)
        )
    }
}

Note: I do not understand the interest of the switch here.
Just

Code Block 
input.indi = i 

should work, isn't it ?

I thought about this but in this way it would change the background of the entire RoundedRectangle and it's not what I want, but Thank you anyway

I guess it is not what you want.

To change this, you need to

• create a view for each element of the picker

• apply the proper rounding to each view

• have a State for each element of the picker (an array)

• apply the .background(didTap ? Color.blue : Color.yellow). to each of the subview in the picker.

So you have some more code to write, but that should not be too hard.

What I want is (for example) when I tap on the Text(arr[0]) at line 12 the background becomes grey and if I tap on Text(arr[1]), Text(arr[0]) returns to be normal background and Text(arr[1]) becomes grey

Why don't you put .background modifier to Text?

Code Block 
struct PickerView: View {
    
    var arr: [String] = ["Easy", "Medium", "Hard"]
    var h: CGFloat = 50
    var w: CGFloat = 320
 
    @Environment(\.colorScheme) var colorScheme
    var normalColor: Color {colorScheme == .dark ? .black : .white}
    
    @ObservedObject var input: UserInput
    var body: some View {
        HStack(spacing: 40){
            ForEach(0..<arr.count) { i in
                HStack(spacing: 25) {
                    Text(arr[i])
                        .bold()
                        .onTapGesture {
                            input.indi = i
                            print(i)
                        }
                        .background(i == input.indi ? Color.gray : normalColor) //<-
                    if i < arr.count - 1 {
                        Divider()
                            .frame(height: 25)
                    }
                }
            }
        }.padding()
        .clipShape(RoundedRectangle(cornerRadius: 16, style: .continuous))
        .overlay(
            RoundedRectangle(cornerRadius: 16)
                .stroke(Color.gray, lineWidth: 3)
        )
    }
}

Why don't you put .background modifier to Text?

Applying modifier to Text will only change the text frame, not the full "cell" of the picker.
I don't think that is the desired effect