When do the application reviewers contact my references (PhD application)? When applying for a PhD position in Germany typically one of the required documents is:

Contact details of at least 2 academic references

This is just contact details (e.g. email addresses) of the people who agreed to act as your reference, not the actual reference letter (the latter is not required).
When do the application reviewers contact my references? Is it before or after they invite me for an interview? Or do they contact my references at all? In other words, in what stage of the application process my references have an impact on the probability of my application being successful?
Admissions committees may contact you references at any time, or not at all. Usually they will look through all applications, then choose a subset to contact references, and a subset of those to invite for interviews. It is also possible that they video interview a subset of applicants then contact their references afterwards.