For editable text fields, users can click into the field and select the data they wish to copy.
For selectable read-only text, as a prerequisite, make sure that this feature is enabled for the table/view you're working with, as explained here. Then, users can select the text with their mouse, 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.