Rewrite a text field in "Manage display" of content type This feels like it should be simple to do, but I can't work it out. 

If I have a text field, how do I rewrite the output of the field on the "Manage display" tab of the content type? For example, I might want to add units, or convert a product ID into a link to an external website. 

I know how to do this stuff in Views, but it would seem to make sense to be able to do it via the "Manage Display" tab for a content type. Am I missing something?
You have a couple of choice based on your needs:


Overriding the template file (twig) of the specific field
Using form_hook_alter
For complex and clean solution I suggest you to use a custom widget for your field type.