join

If you want to join between two or more collection then use lookup() method.

Syntax:-

db.collectionName.aggregate([
    { $lookup:
       {
         from: 'otherCollection',
         localField: 'collection_fieldname',
         foreignField: 'otherCollection_fieldname',
         as: 'collectionDetails'
       }
     }
    ]).toArray();

If you have two collections
1) employees collection which has 3 documents

[
	{
		"_id" : ObjectId("5f26e736deec6e20ea057831"),
		"name" : "John",
		"age" : 35,
		"department" : "department A",
		"salary" : 200000
	},
	
	{
		"_id" : ObjectId("5f26e9dedeec6e20ea057833"),
		"name" : "Tony",
		"age" : 31,
		"department" : "department B",
		"salary" : 40000
	},
	
	{
		"_id" : ObjectId("5f26e9dedeec6e20ea057835"),
		"name" : "Andrew",
		"age" : 33,
		"department" : "department C",
		"salary" : 20000
	}
]

2) department collection

[
	{
		"_id" : ObjectId("5f27bfaddeec6e20ea057836"),
		"name" : "department A",
		"floor" : 5,
		"facility" : "standard"
	},
	{
		"_id" : ObjectId("5f27bfaddeec6e20ea057837"),
		"name" : "department B",
		"floor" : 4,
		"facility" : "good"
	},
	{
		"_id" : ObjectId("5f27bfaddeec6e20ea057838"),
		"name" : "department C",
		"floor" : 3,
		"facility" : "normal"
	}
]

Now you want to get the employees collection document with department details

db.employees.aggregate([
    { $lookup:
       {
         from: 'department',
         localField: 'department',
         foreignField: 'name',
         as: 'departmentDetails'
       }
     }
    ]).toArray();

Output:-

[
	{
		"_id" : ObjectId("5f26e736deec6e20ea057831"),
		"name" : "John",
		"age" : 35,
		"department" : "department A",
		"salary" : 200000,
		"departmentDetails" : [
			{
				"_id" : ObjectId("5f27bfaddeec6e20ea057836"),
				"name" : "department A",
				"floor" : 5,
				"facility" : "standard"
			}
		]
	},
	
	{
		"_id" : ObjectId("5f26e9dedeec6e20ea057833"),
		"name" : "Tony",
		"age" : 31,
		"department" : "department B",
		"salary" : 40000,
		"departmentDetails" : [
			{
				"_id" : ObjectId("5f27bfaddeec6e20ea057837"),
				"name" : "department B",
				"floor" : 4,
				"facility" : "good"
			}
		]
	},
	
	{
		"_id" : ObjectId("5f26e9dedeec6e20ea057835"),
		"name" : "Andrew",
		"age" : 33,
		"department" : "department C",
		"salary" : 20000,
		"departmentDetails" : [
			{
				"_id" : ObjectId("5f27bfaddeec6e20ea057838"),
				"name" : "department C",
				"floor" : 3,
				"facility" : "normal"
			}
		]
	}
]