First things first. Paul, you are awesome! I never thought, that I start programming on XCode ever... Love your videos and the way you make things clear. Even as a non-native english speaker, I can follow your descriptions. Here is my solution:

//
//  ContentView.swift
//  Converter Liquids
//
//  Created by Oliver Kleemann on 05.04.22.
//

import SwiftUI

struct ContentView: View {

    @State private var from: String = "Liter"
    @State private var to: String = "Liter"
    @State private var value: Double = 1.0

    // add more units into this dictionary here:
    let units = ["Liter": 1.0,
                 "Milliliter": 1000.0,
                 "Hektoliter": 0.01,
                 "Kubikmeter": 0.001,
                 "US-Gallone": 0.264172,
                 "US-Quart": 1.05669,
                 "US-Pint": 2.11338,
                 "US-Cup": 4.16667,
                 "UK-Gallone": 0.219969,
                 "UK-Quart": 0.879877,
                 "UK-Pint": 1.75975,
                 "UK-Cup": 3.51951]

    // this is the array used for the both pickers
    var unitArray: [String] {
        var outputArray: [String] = []
        for unit in units { outputArray.append(unit.key) }
        return outputArray
    }

    var conversionResult: Double {
        let fromFactor = units[from] ?? 0.0
        let toFactor = units[to] ?? 0.0
        return value / fromFactor * toFactor
    }

    var body: some View {
        NavigationView {
            Form {
                Section {
                    Picker("From unit:", selection: $from) {
                        ForEach (unitArray.sorted(), id: \.self) {
                            Text ($0)
                        }
                        .pickerStyle(.automatic)
                    }

                    TextField ("Value to convert", value: $value, format: .number)
                        .keyboardType(.decimalPad)
                } header: { Text("Input") }

                HStack{
                    Spacer()
                    // exchange the units
                    Button ("↕️"){
                        let exchange = from
                        from = to
                        to = exchange
                    }
                    Spacer()
                }

                Section {
                    Picker("To unit:", selection: $to) {
                        ForEach (unitArray.sorted(), id: \.self) {
                            Text ($0)
                        }
                        .pickerStyle(.automatic)
                    }

                    Text (conversionResult, format: .number)
                } header: { Text ("Result") }
            }
            .navigationTitle("Liquid Converter")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .previewInterfaceOrientation(.portrait)
    }
}

Took me a long time, but with the knowledge of WeSplit, the goal was reachable. A lot to learn ...

Thanks for your great work!

Nice job. Congrats on hanging in there and solving the challenge. In case you want a little less typing, this is a shorter way for creating an array of keys from a dictionary:

var unitArray: [String] { Array(units.keys) }

Thanks :) I already had the feeling, that there is a better way to generate this Array. Isn't there maybe a way to go through the keys directly in the ForEach statement? Without the Array?

Yes, you can go even futher and remove unitArray; and use this code in your ForEach instead:

ForEach(units.keys.sorted(), id:\.self) {

Wonderful! Thank you.