Understanding of FHIR QuestionnaireResponse

I understand the FHIR Questionnaire Response instance to be a completly answered Questionnaire. (Is that even correct or can a Questionnaire Response also be an incomplete answered Questionnaire?). Is the Questionnaire Response meant to be a single source of truth or is it also possible to use references to for example observations to catch the answers?
Kind Regards,

A QuestionnaireResponse could be partially answered - it depends on the status. Also, a ‘complete’ QuestionnaireResponse might still have some answers missing if those questions happen to be optional.

Questionnaires and their responses are used to represent raw data capture where there’s tight control over exactly how questions are asked, what order they’re asked in, what answers are allowed, etc. Typically the information gathered in a QuestionnaireResponse will then be extracted into Observations, AllergyIntolerance, Procedure, FamilyMemberHistory, and other resources that are consistent representations independent of how the data was gathered and that support standardized search and retrieval.

Thanks for your reply!
Regarding the extraction of information, is it possible to do it the other way around? To insert the answers for the Questionnaire Response from information collected in other resources by using their reference as answer value or have the answers in the Questionnaire Response to be concrete values?

Absolutely. That process is called ‘population’. There’s guidance on both (plus a whole lot more Questionnaire-related stuff) in HL7’s Structured Data Capture (SDC) IG