Modifying buildsteps/artefactpaths in a templated build (Kotlin)

We've used TeamCity for 10 years or so, and have only just started moving our configurations to Kotlin (primarily because we want different builds for different branches).

When do templated builds "add" vs "replace" things defined in the base template? In particular, I want to add a buildstep and some additional artefacts rules to those already defined in the template.

If I define artifactRules in the (derived) build, will this replace or append to the artifactRules already defined in the template? How do I make it append?

Thanks

Rob