How do I override a view template in my theme folder properly? I do not know if I am something missing, but I have a View in Drupal 8 and wanted to make a Template override in my theme, where I would print-out the image URL of one field and set it as "background-image" via inline CSS.

So I turned on theme debugging to see the template suggestions and picked views-view-field.html.twig, copied the file to my theme, renamed it to match the suggestion, cleared the cache and it looks like the template override is working.

But all the variables I am trying to print-out are blank, even after clearing the cache the field just disappeared. When I try to dump() or kint() any variable written in the file (view, field, row, output) it prints "null" or completelly nothing.

What am I doing wrong?

I am sure template override works, because when I type regular text in a file it gets printed out, but all the variables are somehow missing and I can not print anything out of them.
Copy the content of the previous template used and start from there.
It is most likely that the variable you are trying to display are not available in that template.
Consider adding variables using hook_preprocess.