Basic Filtering

You already call find and findOne. This module is the filter — the first argument. A filter is just a document. MongoDB keeps a document if every field in the filter matches.

db.users.find({ city: "Bangalore" })

// same meaning
db.users.find({ city: { $eq: "Bangalore" } })

{ name: "Rahul", city: "Bangalore", isActive: true, ... }
{ name: "Priya", city: "Bangalore", isActive: true, ... }

Two keys in one filter are an implicit AND. Both must match. You do not need $and for this.

db.users.find({
  city: "Bangalore",
  role: "developer",
  isActive: true
})

SELECT * FROM users
WHERE city = 'Bangalore'
  AND role = 'developer'
  AND isActive = true;

• {} matches every document. That is find() with no filter — fine for a tiny collection, a bug in production.
• A missing field does not equal false or null. { isActive: false } will not return users who have no isActive at all. $exists is two lessons away.
• Types must match. { age: "28" } does not hit age: 28.