private GroupAssignment assignHeterogeneous(
        GroupSpec groupSpec,
        SubscribedTopicDescriber subscribedTopicDescriber
    ) {
        Map<String, MemberAssignment> members = new HashMap<>();
        for (String memberId : groupSpec.memberIds()) {
            MemberSubscription spec = groupSpec.memberSubscription(memberId);
            if (spec.subscribedTopicIds().isEmpty())
                continue;

            Map<Uuid, Set<Integer>> targetPartitions = computeTargetPartitions(
                spec.subscribedTopicIds(), subscribedTopicDescriber);

            members.put(memberId, new MemberAssignmentImpl(targetPartitions));
        }
        return new GroupAssignment(members);
    }
A simple partition assignor that assigns each member all partitions of the subscribed topics.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/SimpleAssignor.java