Where is the cube to charge my Apple watch

See the following from (just one example) https://www.apple.com/shop/buy-watch/apple-watch?option.watch_cases=MKNQ3LL/A&option.watch_bands=ML7Y3AM/A&preSelect=false&product=Z0YQ&step=detail#

What’s in the Box

Green Aluminum Case

Leather Link

1 Magneti Fast Charger to USB‑C Cable

As part of our efforts to reach our environmental goals, Apple Watch does not include a power adapter. Included in the box is a Magnetic Fast Charger to USB‑C cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.

If you don't have any USB-C power source, the following is one option commonly used: https://www.apple.com/shop/product/MHJA3AM/A/20w-usb-c-power-adapter

With your phone charger and an old watch charging puck,you won't get fast charging though.