Enterprise test> db.a.aggregate({$lookup: {from: "a", localField: "b", foreignField: "b", as: "x", pipeline: [{$project: {b: 1}}]}}, {$unwind: "$x"}).explain().queryPlanner.winningPlan
{
  isCached: false,
  usedJoinOptimization: true,
  queryPlan: {
    stage: 'NESTED_LOOP_JOIN_EMBEDDING',
    planNodeId: 4,
    costEstimate: 0.19359127036905283,
    cardinalityEstimate: 1,
    estimatesMetadata: { ceSource: 'Metadata' },
    leftEmbeddingField: 'none',
    rightEmbeddingField: 'x',
    joinPredicates: [ 'b = b' ],
    inputStages: [
      {
        stage: 'COLLSCAN',
        planNodeId: 1,
        costEstimate: 0.09602483518452641,
        cardinalityEstimate: 1,
        estimatesMetadata: { ceSource: 'Metadata' },
        filter: {},
        nss: 'test.a',
        direction: 'forward'
      },
      {
        stage: 'PROJECTION_SIMPLE',
        planNodeId: 3,
        costEstimate: 0.09634243518452641,
        cardinalityEstimate: 1,
        estimatesMetadata: { ceSource: 'Metadata' },
        transformBy: { _id: true, b: true },
        inputStage: {
          stage: 'COLLSCAN',
          planNodeId: 2,
          filter: {},
          nss: 'test.a',
          direction: 'forward'
        }
      }
    ]
  },
  slotBasedPlan: {
    slots: '$$RESULT=s8 env: {  }',
    stages: '[4] project [s8 = makeBsonObj(MakeObjSpec([x = Set(0)], Open, NewObj, 0), s1, s4)] \n' +
      '[4] nlj inner [s1, s3] [] {((s3 ?: null) == (s7 ?: null))} \n' +
      '    left \n' +
      '        [1] scan generic [s1 = record, s2 = recordId] [s3 = b] @"293be7e7-6dea-4723-bd39-54ba46fa6922" \n' +
      '    right \n' +
      '        [2] scan generic [s4 = record, s5 = recordId] [s6 = _id, s7 = b] @"293be7e7-6dea-4723-bd39-54ba46fa6922" \n'
  }
}