I am new to web development and I am currently implementing a web application using .net core and angular where I need to store some of the data encrypted (using AES) in a mongoDB database.
I don't want to encrypt all the fields of my model, only 2 or 3 fields. What would be a good approach to achieve this please.
You'll have to first decide where the encryption will take place, client or server. If it's done on the server, you have to make sure you are using SSL or another encrypted channel between client and server.
Since your client model and database (MongoDB) model are different (one encrypted and one not), you should have 2 different model classes to represent that. Then have a mapper class to convert from one to the other. The mapper class is what handles the encryption (perhaps through another class dependency that is dedicated to encryption)