Most apps support copy-and-paste, but not all. Touch and hold the area that you want to copy, if the app supports the feature a pop-up box will appear that offers 'Select', 'Select All' and so on. If the entire field is what you're after, choose 'Select All'. Then another pop-up box will appear - 'Cut', 'Copy'. Choose 'Copy'.
Now go to the target app. Press and hold where you want to insert the copied text. A pop-up box will appear and 'Paste' will be one of the options.
If you don't get the first set of pop-up boxes in your calculator app, then it's because the Developer has chosen not to implement this feature.
Tim