Honest note: no phone app replaces desktop CAD/BIM for delivery work — apps shine at sketching, site capture and showing models. For the free desktop stack, see free architecture software; for what employers actually ask for, browse the live BIM & Revit jobs.
More guides: free architecture software · SketchUp for free · free floor plan software · salary calculator.
It depends on the task: Morpholio Trace for sketching over plans on iPad, BIMx or the Autodesk Viewer for showing models on site, and magicplan or Polycam for capturing a space with your phone.
Not delivery work — mobile apps shine at sketching, viewing and site capture. For drafting and BIM you still need desktop software; several genuinely free options exist.