How can i copy one column from one collection to another collection in mongoDB using Python?
document:{
  _id: Object_ID
  username: "john"
  email: "john@email.com"
  age: 24
}
Now, you want to copy username column from this document of some collection1 to a document of collection2, for that,
let say you have build connection with MongoDB, and we use db object to further data manipulation.

we need to get that intended column data with column name
data = db.collection1.find_one(
  {"email": "john@email.com"},
  {"username": 1}
)
// now data is a python dictionary contains value of username on key username
now we are going to create a new document in another collection(or we can update that data based on some condition if you wish)
db.users.collection2.insert_one(data) //hence we got our new document with that data in collection2

Now here I use find_one to exactly collect a single data, but if you need to collect a list of data then, just iterate through the list while inserting those data into a collection.
Adding column in different collection