Write code with tests in mind
You can write code in many different ways, some of them are more testable than other, even though many of the less testable ways are not bad or anyting like that. Like mutating a reference value like when something takes a output pointer etc. Or having most of the code in the controller.
None of these are “bad”. But it makes it harder to write unit tests for it.
If you think something is good, then try to make it as easy as possible to do it.
I don’t do test driven development, because I don’t think it is the best way to work. But I do think that writing unit and integration tests should be a part of the process, because having good tests and coeverage will save you time in the long run. And it is a must if you want to move to the next level and get into automated deployments.
If you start with a typical C# .NET Core REST API. Then you could do everything in the Controller. It would work well. And the code could be clean. Probably a little or a lot of duplication of code but still it could work.
The problem with this approach however is that there is nothing you can unit test, because everything is tied together. Then you are limited to integration tests.
What I usually do is that I take the Controller and I limit it to validating input, checking permissions (ideally done with attributes and filters) and calling a “service”.
The services takes input from the controller and is in charge of checking all the right things, doing all the correct operations and preparing the data structure the API should return.
Then all SQL stuff, and all the Redis stuff etc are put into repositories. And the classes that takes data and turn into something else are done in builder classes.
All of this are tied together with as many static methods as possible (where it is possible) and dependency injection.
The advantage of this approach is that you can actually write tests for the controller without it showing up in the database. And the same goes for the service and the builders. Because you can just fake the dependency injection crap.
If you want to have a project where most of the code is covered by tests, then you need to write the code in a way where it is possible.