What a pipeline actually is

A pipeline is a list of stages, and each stage receives whatever the previous stage emitted. Nothing more mysterious than that - but the “whatever the previous stage emitted” part is where every aggregation bug lives.

db.orders.aggregate([
  { /* stage 1 */ },
  { /* stage 2 */ },
  { /* stage 3 */ }
])

Think like Node.js:

const result = orders
  .filter(...)
  .map(...)
  .sort(...)

Aggregation is conceptually similar:

$match    ≈ filter()
$project  ≈ map()
$sort     ≈ sort()
$group    ≈ groupBy + reduce()

That analogy is not perfect, but it is useful.

Try it

The same three stages, filled in:

db.orders.aggregate([
  {
    $match: { status: "completed" }
  },
  {
    $project: { _id: 0, userId: 1, status: 1 }
  },
  { $limit: 5 }
])

Delete the $match and run it again. The $project did not change, but what it received did - and that is the whole of aggregation.