Technically, both forms are interchangeable in both these examples. I know the code catches errors.

In my opinion the distinction here comes mainly because “main” is usually a stronger or more emphatic version of “in”. As such, “in” is usually used when the enclosing object often has things going into and out of it, and the question of what is in it might come up on a regular basis. Inside adds emphasis for situations where one might not normally expect the one thing to be inside the others, or it’s not as common to consider what might or might not be Inside of it.

So therefore, we would usually say something ‘in the house’ but we would tend to say something ‘inside his body’.

There are other distinctions that come into play. What is meant by inside “in” in “manly ways” usually mean that a part, while other parts have a bigger whole (“there is garlic and broccoli and asparagus in soup”) is divided into two parts, and one just has to fit with the other (“the filling is inside a cupcake”). What has been “obtained in inside” also sometimes has an implication that something is not only in something else, but that the containing object has been closed up around it, or made it harder to get to.

Lastly, it’s important to note that “inside” can be used by itself when one doesn’t need to specify the noun explicitly (“Fred is inside. “), whereas “in” cannot.