Mongo is the most popular database of choice in the NoSQL sector. Mongo offers unique advantages in terms of fast-changing business requirements in data modeling
Mongo is not suited for every use case, but it does offer significant advantages when entities are not very tightly coupled.
Where there is Dynamic / Unknown size and structure of data choose mongo.
We generally use MongoDB with mongoose ORM in expressjs projects. Express and Mongoose will be introduced in later chapters but you still need solid foundations in Mongo & aggregation pipelines to really use them to the fullest.
Best resource to learn MongoDB is this course https://university.mongodb.com/courses/M001/about