How do you make a list and list item relation if custom post types?

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Here are few ideas how to relate content together in WordPress. Without knowing (and understanding) all the details and requirements it’s difficult to give a complete answer, but these hopefully help you explore different possible solutions.

  1. Store relation data in CPT post meta. You can then use WP_Query with
    meta parameters to find matching posts
  2. Manage relations with post_parent property of WP_Post (it has its own column in DB {prefix}_posts table). Nothing stops you from relating different posts types together like this, if you handle the data saving yourself with wp_update_post()
  3. Create a custom taxonomy to serve as lists and use CPT as items
  4. Use CPT as list and store list items as serialized array / json in the post’s post_content property
  5. Skip CPTs altogether, store everything in custom DB tables and manage data with $wpdb

P.s. please note that these kind of open questions usually don’t have a single definite answer, but recieve many opinion-based answers each with many ifs, buts and maybes, and your question might get closed.