How do I display sections such as “Customers Also Bought” or “You May Also Like” on product pages?

Go to Preferences under Online Store in the Settings page. You can enable three recommendation types, each with its own Maximum Recommendation Count (default: 3):

Related Items: Items similar to the one being viewed. Shown on the item details page.
Upsell Items: Higher-value or upgraded versions of the item. Shown on the item details page.
Cross-sell Items: Complementary items that pair with it. Shown on the cart details page.

Use the Select Recommendation Preference dropdown to choose how items are sourced (Manual, Rule, Automatic, or Manual, Rule). Select the recommendation types you need, set each count, and click Save.