How does a string catalog (.xcstrings) work, and which runtime files does the build compile it into?

Predict first

What files does a catalog with two languages and a plural compile into?

import Foundation

let fm = FileManager.default
let root = fm.temporaryDirectory.appendingPathComponent(UUID().uuidString)
defer { try? fm.removeItem(at: root) }
try fm.createDirectory(at: root.appendingPathComponent("out"), withIntermediateDirectories: true)
let json = """
{ "sourceLanguage" : "en", "version" : "1.0", "strings" : {
  "Hello" : { "localizations" : { "fr" : { "stringUnit" : { "state" : "translated", "value" : "Bonjour" } }, "de" : { "stringUnit" : { "state" : "needs_review", "value" : "Hallo" } } } },
  "%lld files" : { "localizations" : { "en" : { "variations" : { "plural" : { "one" : { "stringUnit" : { "state" : "translated", "value" : "%lld file" } }, "other" : { "stringUnit" : { "state" : "translated", "value" : "%lld files" } } } } } } },
  "Old" : { "extractionState" : "stale", "localizations" : { "fr" : { "stringUnit" : { "state" : "translated", "value" : "Vieux" } } } }
} }
"""
let src = root.appendingPathComponent("Localizable.xcstrings")
try json.write(to: src, atomically: true, encoding: .utf8)
let p = Process()
p.executableURL = URL(fileURLWithPath: "/usr/bin/xcrun")
p.arguments = ["xcstringstool", "compile", src.path, "-o", root.appendingPathComponent("out").path]
try p.run(); p.waitUntilExit()
let en = fm.enumerator(at: root.appendingPathComponent("out"), includingPropertiesForKeys: nil)!
for case let u as URL in en where !u.hasDirectoryPath {
    let rel = u.pathComponents.suffix(2).joined(separator: "/")
    let d = NSDictionary(contentsOfFile: u.path) ?? [:]
    print(rel, d.allKeys.map { "\($0)" }.sorted())
}
let fr = NSDictionary(contentsOfFile: root.appendingPathComponent("out/fr.lproj/Localizable.strings").path)!
print("fr Old (stale) still compiled:", fr["Old"] as? String ?? "no")
let de = NSDictionary(contentsOfFile: root.appendingPathComponent("out/de.lproj/Localizable.strings").path)!
print("de needs_review compiled:", de["Hello"] as? String ?? "no")

• de.lproj/Localizable.strings ["Hello"] / en.lproj/Localizable.stringsdict ["%lld files"] / fr.lproj/Localizable.strings ["Hello", "Old"] / stale shipped / needs_review shipped
• one file only
• fr has only Hello