no indexes on the foreign collection "r", NLJ is used:
db.l.aggregate({$lookup: {from:"r", localField:"b", foreignField:"a", as:"matched"}})
{ "_id" : 0,
"b" : [ 1, 2 ],
"matched" : [

{ "_id" : 2, "a" : [ 1, 2 ], "y" : 3 }

,

{ "_id" : 11, "a" : [ 2, 1 ] }

]
}

db.r.createIndex(

{a:1}

) // this enables INLJ strategy for the lookup
db.l.aggregate({$lookup: {from:"r", localField:"b", foreignField:"a", as:"matched"}})
{ "_id" : 0,
"b" : [ 1, 2 ],
"matched" : [

{ "_id" : 2, "a" : [ 1, 2 ], "y" : 3 }

,

{ "_id" : 11, "a" : [ 2, 1 ] }

,

{ "_id" : 2, "a" : [ 1, 2 ], "y" : 3 }

,

{ "_id" : 11, "a" : [ 2, 1 ] }

] }