How to define Views filters and relationships for custom Civicrm contact fields that are indexed via Search API?
If this is about getting Membership fields showing in the ADD RELATED FIELDS selector at the bottom of admin/config/search/search_api/index//fields then I expect this is a civicrm entity issue in which case you could check at this chat channel

Otherwise you may be looking for sites/all/modules/civicrm/drupal/modules/views