have you check the documentation? About half the page starts the examples.
$lookup (aggregation) — MongoDB Manual
It would be better if you give 2-3 sample documents of each collection and your expected output from them. Replace all ObjectId with a number or simple string: {_id:123} or {_id:"abc"} (flexible types). Also add 1-2 more fields if you are not doing full merges. This will enable us to work on real (sample) data and find a solution faster.