where do we write code to insert or update database. In Modal or controller

You have raised a very good question and an ambigous one. The main focus is to make our application loosely coupled, else if it is tightly coupled then we have to compromise on many things. SO, better follow the
"REPOSITORY PATTERN"

What is this??
Now We have models, that is our classes for the project. We have controllers, but remember never use DB operations in the Controllers, its bad. It makes tight coupling. So have "Service Methods" & "Repositories" they will handle all the DB operations i.e. in coding language The Context will play its role in the Services/Repositories.
Use them creating the constructors for the services interfaces in the controller and access them.
Remember a main thing here to understand is Dependency injection that handles the Parameterless Constructor issue. Please read about Dependency Injection. This will surely help you do good coding.

Thanks.

You write the code inside Model. Model contains classes, validations and operations. Controller interacts with the View-Model (in this case calls to your Database operation).

For example, you can see this link[^].

I wrote a wrong comment :(

Thanks to @CHill60 and @Wes Aday for their comments.

It depends,

I follow different strategy, In model I only writes static classes, which I generally use for dynamic partial views and direct calling the methods without intimating the controller. and In controller I write the methods to do all the other functionality like inserting the data in the database getting the value from the database (in sort CRUD).