> There are no definitions, no guidelines.
There are both definitions and guidelines. You just didn't even care to read it.
For example, cookies: https://gdpr.eu/cookies/
Popups: https://gdpr.eu/recital-32-conditions-for-consent/
=== start quote ===
Consent should be given by a clear affirmative act establishing a freely given, specific, informed and unambiguous indication of the data subject’s agreement to the processing of personal data
...
Silence, pre-ticked boxes or inactivity should not therefore constitute consent.
=== end quote ===
> there is no consensus on simple questions like website analytics which is probably the most common usage scenario for the kind of data GDPR covers.
The consensus is there. And it's spelled clearly in the law.