In the modern experience, the detail information panel (right panel) for displaying properties of an item does not allow users to select any content in that. Thus, you cannot select and copy any thing in detail information panel. Users can only copy content in edit mode.
To work around this, you can switch to classic experience by clicking “Return to classic SharePoint” in the bottom, left corner of the page to switch to classic experiences. Users can copy field values including multiple-lines of text field values in classic experience.
By the way, you can submit your feedback to SharePoint UserVoice for this requirement as the modern experience is a new feature and is in the improving phase. If a feedback is high voted there by other customers, it would be promising that Microsoft Product Team will take it into consideration when designing the next version in the future.