How can I export my customers and know what lists they are a part of?
The easiest way to do this is to use the update profile property action functionality using a flow. To start, create a flow triggered by someone being added to each list in your account. Once you have created the flow trigger, add in an "updated profile property" action. You can create a profile property called List Membership and then assign a value of the List name. For more information on how to do this, head to this article on Updating Profile Property Action.
Your next step is to back-populate the flow by selecting (for more information on this, head to our article on how to Back-Populate a Flow). Choose the second option to "schedule emails relative to when I hit the back-populate button." As long as you only add in the updating profile property action, no emails will send out because your flow does not contain any emails. This will send all customers in your list through the flow to add the custom profile property to their user profile in Klaviyo. You can then create segments of customers or export your individual lists and see which lists your customers are a part of by looking at the custom profile property values.