Can I split my Pulumi code into seperate classes/files (in dotnet)?

is https://www.pulumi.com/learn/building-with-pulumi/stack-references/ what you mean?

I’m not familiar with .NET (are you using C#?), but if the language allows you to put portions of code in separate files and then import/reference those files, then you can absolutely use this functionality when writing a Pulumi program.