Where can I find the URL to the questionnaire? What about the QR code?
Can I customize the URL?
Yes, you can edit it in Publish > Customize > Custom URL.
Will the URL change if I publish the questionnaire again in test mode or when I publish it for the final time?
No, the URL will stay the same.
Will the QR-code change if I customize the URL
Yes, Custom URL changes what the QR code points to. This is because the QR-code is generated by reading whatever text string sits in the URL field and converting that string into a scannable pattern.
Is it possible to send unique links to each respondent?
Yes, you can, please see Track the link respondents use to access the survey (Panel tracker)
Can I track how respondents accessed the survey?
Yes, see these two articles:
Track the link respondents use to access the survey (Panel tracker)