There are well-documented best practices for website feedback collection among non-converters on desktop. For example, you can trigger a survey to launch on Exit Intent if the user's site history does not include the order confirmation page. In the survey you can then ask questions about the UX.
On the mobile (non-app) side, I see challenges to collecting this type of feedback:

• You can trigger a survey on the mobile site at any time. But, you really want to collect feedback at the end of a user's session (so that they can comment on the full experience and so that you do not interrupt their shopping experience). However, there is no "On Exit Intent" trigger on mobile.


• Qualtrics allows you to prompt a user during his/her session to enter email address so that you can send a UX survey via email following the session. This approach interrupts the browsing experience and, I imagine, yields very low response rates.

How do other websites handle UX feedback collection mobile?
A "Feedback" or "Share Your Opinion" button on the side of the page gives you feedback mid-session (not at the end), and I am concerned that users have come to see these buttons as a way to reach out to customer service or to complain, not to give unbiased answers to survey questions.
A responsive dialog pop-up or slider bar is also mid-session and seems quite disruptive to the user experience.
I don't see many UX surveys on mobile, maybe these complications explain why...