For editable fields, users can simply click into the field and select the data they wish to copy.
For read-only fields, as a prerequisite, make sure that this feature is enabled for the table/view you're working with, as explained here. Then, have users triple-click into the field to select the entire content, after which they can use their computer's copy functionality (e.g. CMD+C, CTRL+C, right-click -> Copy) to copy the data. They can also highlight a portion of the data just like text in other applications.