demo code for laying out a SwiftUI table with fitted columns

TableDemo.swift

import SwiftUI

fileprivate struct WidthPreferenceKey: PreferenceKey {
    static var defaultValue: [AnyHashable: CGFloat] { [:] }

    static func reduce(value: inout [AnyHashable : CGFloat], nextValue: () -> [AnyHashable : CGFloat]) {
        value.merge(nextValue(), uniquingKeysWith: { max($0, $1) })
    }
}

extension WidthPreferenceKey: EnvironmentKey { }

extension EnvironmentValues {
    fileprivate var widthPreference: WidthPreferenceKey.Value {
        get { self[WidthPreferenceKey.self] }
        set { self[WidthPreferenceKey.self] = newValue }
    }
}

extension View {
    public func widthPreference<Domain: Hashable>(_ key: Domain) -> some View {
        return self.modifier(WidthPreferenceModifier(key: key))
    }
}

fileprivate struct WidthPreferenceModifier: ViewModifier {
    @Environment(\.widthPreference) var widthPreference
    var key: AnyHashable

    func body(content: Content) -> some View {
        content
            .fixedSize(horizontal: true, vertical: false)
            .background(GeometryReader { proxy in
                Color.clear
                    .preference(key: WidthPreferenceKey.self, value: [key: proxy.size.width])
            })
            .frame(width: widthPreference[key])
    }
}

public struct WidthPreferenceDomain<Content: View>: View {
    @State private var widthPreference: WidthPreferenceKey.Value = [:]
    private var content: Content

    public init(@ViewBuilder content: () -> Content) {
        self.content = content()
    }

    public var body: some View {
        content
            .environment(\.widthPreference, widthPreference)
            .onPreferenceChange(WidthPreferenceKey.self) { widthPreference = $0 }
    }
}

struct ContentView: View {
    var body: some View {
        VStack(spacing: 0) {
            ZStack {
                TitleBackground()
                Text("Clients")
                    .foregroundColor(.white)
            }
            TableView()
            Spacer()
        }
    }
}

enum ColumnId: Hashable {
    case name
    case balance
    case currency
}

struct TableView: View {
    var body: some View {
        WidthPreferenceDomain {
            VStack(spacing: 0) {
                HStack(spacing: 30) {
                    Text("Name")
                        .widthPreference(ColumnId.name)
                    Text("Balance")
                        .widthPreference(ColumnId.balance)
                    Text("Currency")
                        .widthPreference(ColumnId.currency)
                    Spacer()
                }
                .frame(maxWidth:.infinity)
                .padding(12)
                .background(Color(#colorLiteral(red: 0.9332349896, green: 0.9333916306, blue: 0.9332130551, alpha: 1)))
                ForEach(0 ..< 4) { _ in
                    RowView()
                }
            }
        }
    }
}

struct RowView : View {
    var body: some View {
        HStack(spacing: 30) {
            Text("John")
                .widthPreference(ColumnId.name)
            Text("$5300")
                .widthPreference(ColumnId.balance)
            Text("EUR")
                .widthPreference(ColumnId.currency)
            Spacer()
        }
        .padding(12)
    }
}

struct TitleBackground: View {
    var body: some View {
        ZStack(alignment: .bottom){
            Rectangle()
                .frame(maxWidth: .infinity, maxHeight: 60)
                .foregroundColor(.red)
                .cornerRadius(15)
            //We only need the top corners rounded, so we embed another rectangle to the bottom.
            Rectangle()
                .frame(maxWidth: .infinity, maxHeight: 15)
                .foregroundColor(.red)
        }
    }
}

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

It works, but few suggestions

1. I would remove:
   .fixedSize(horizontal: true, vertical: false)

that would make columns freely break into multiple lines.

2. I would add alignment as an optional parameter as sometimes you need leading alignment or trailing:

extension View {
    public func widthPreference<Domain: Hashable>(_ key: Domain, alignment: Alignment = .center) -> some View {
        modifier(WidthPreferenceModifier(key: key, alignment: alignment))
    }
}

fileprivate struct WidthPreferenceModifier: ViewModifier {
    @Environment(\.widthPreference) var widthPreference
    
    var key: AnyHashable
    
    var alignment: Alignment
    
    func body(content: Content) -> some View {
        content
            .background {
                GeometryReader { proxy in
                    Color.clear
                        .preference(key: WidthPreferenceKey.self, value: [key: proxy.size.width])
                }
            }
            .frame(width: widthPreference[key], alignment: alignment)
    }
}

3. One issue that I encountered while testing is what to do when data changes? If data changes layout won't always update? How to fix this?