In what source file do actors get loaded into editor?
You’ll have to be more specific.
Which Actors? Actors in your current map?
Yes , actors for current map.
Would like to try spawning additional actors to the editor.
There is an Editor delegate called FEditorDelegates::OnMapOpened that you can likely use. Seems like HLOD and Sequencer are already using it to do something similar. Just search the code for OnMapOpened and you’ll find some examples.
Could it work by placing the delegade in project sources and spawning actors to editors viewport/object list from there or project code gets executed only during gameplay and other way like plugin etc. needs to be used?