public Map<String, FieldSet> builtInFieldSets() {
        if (owner.isEmpty() || owner.get().inherited().isEmpty()) return Collections.unmodifiableMap(builtInFieldSets);
        if (builtInFieldSets.isEmpty()) return owner.get().inherited().get().fieldSets().builtInFieldSets();

        var fieldSets = new LinkedHashMap<>(owner.get().inherited().get().fieldSets().builtInFieldSets());
        fieldSets.putAll(builtInFieldSets);
        return Collections.unmodifiableMap(fieldSets);
    }
Returns the built in field sets, unmodifiable
config-model/src/main/java/com/yahoo/schema/FieldSets.java