Hi Wojtek,
You can cycle trough dense clouds with something like this:
import Metashape
chunk = Metashape.app.document.chunk
for each in chunk.dense_clouds:
print(each.label)
and choose the default with something like this:
import Metashape
chunk = Metashape.app.document.chunk
chunk.dense_cloud = chunk.dense_clouds[-1] #This selects the last dense cloud,, but you can use whatever index suits you.
chunk.dense_cloud.label = "Hello there." #just to prove it works
The same applies to chunk.model and chunk.models
Hope this helps!