Does QGIS use data source-specific operations?

This may be a very basic question, but I couldn’t find an appropriate answer.

Does QGIS use data source specific operations where available and appropriate? For example, when using the Join attributes by location dialogue to join two layers within the same PostGIS database, will QGIS create an appropriate SQL query and run that query on the PostGIS server? Or will QGIS fetch all relevant features and do the join locally?

I know that I can write my own queries through plugins and/or DB manager, just interested in the default behaviour?