Can I split my Pulumi code into seperate classes/files?

Yes. My Pulumi project defines resources in 5 separate classes/files, invoked from the primary class.

Do you have a code sample? Is it basically just copy the main class and the code in that?

The Program file looks like this:

class Program
{
    static Task<int> Main() => Deployment.RunAsync<ApiResourcesStack>();
}

The constructor of

ApiResourcesStack

(the class I referred to as "primary") instantiates the classes defined in other files, e.g.

var sqlResources = new AzureSqlResources(rgName, envSuffix, envTagName);

Typically, the object returned from one of those

new <class>

calls has properties passed to a subsequent

new <class>

call. That primary file does not itself define/create any resources - it relies on the other classes. But that primary class does handle the

[Output]

properties. Of course, this is just the way that I've chosen to do it. There are assuredly other ways.

How does your second cclass look?

Either directly in those classes' constructors, or in static helper functions within those classes, they just use standard Pulumi code, i.e.

new <PulumiResourceClass>("name", new <PulumiResourceClassArgs>
{
    // set properties
});

IOW, basic C# using the Pulumi constructs, types, API, etc.

It's also perfectly valid to use a static method if you want.