How do I create a custom view filter for an existing field? I ran into a problem with creating a custom view filter for an existing field for Drupal 8. I mean a filter not for fields like "nid", "title" and so on, but for fields, which were named CCK-fields first.

Could you share with me an understandable example of code, how to create this plugin? 

I tried to comprehend some examples in contrib modules, like the geolocation module, but that code was very sophisticated. Also, I haven't found adequate manuals anywhere.
If you do not need to code your Views filter, in your Articles View, you have a Filter criteria section where you may add views filters based on many different entities, including fields (what you call CCK fields)￼.