Manipulate a render array in hook_node_view
Suggestions


Get the node from the $build (it should be in there somewhere) and build custom content for your field like $build['custom_field'] = [...].
Override the twig template for the field - this will let you alter the presentation of a particular field. Ex: implement node--field-custom-field.html.twig based on field.html.twig and alter the output of the field there.
Override the twig template for the entire node. Ex: implement node--article.html.twig based on node.html.twig and alter the output of the node there.


Let me know if any of these helps. More details on exactly what or why you want to alter stuff will be helpful.