How do I show "You may also like" products on a product page or in the cart?

Use the Up / Cross Sell plugin. It has two parts:

• Upsell: related products shown on the product page.
• Cross-sell: related products shown in the cart.

1. Activate the plugin, then open Plugins → Up / Cross Sell.
2. On the Upsell and Cross-sell tabs, tick Enable, set the Heading, Products to show (0 shows all) and the Placement (for example Product page — bottom (default) or Cart — after items). Click Save.
3. Open a product. In the Upsell & cross-sell card, pick its Upsell products and Cross-sell products.

Default products on each tab are used when a product has none of its own. To move or hide the block, use Plugins → Hook Manager.