My Apple Watch came with a magnetic round device and cord but no adapter to allow me to charge the watch. How do I charge it

You buy a USB C charging brick, and to use fast charging you buy one rated at 20W. Apple sells them. Best Buy sells them. Amazon sells them.

Or, if your computer has USB C output, you can plug the puck into your computer too.

Many new Apple products do not come with the "power brick" anymore. It is assumed we all have a bunch of them lying around and/or you will buy it separately or already have some other way of charging your devices. For example, you can use your computer to charge your Apple devices. If you have other devices that have either USB-A or USB-C ports on them, chances are you can use them to charge your Apple Watch as well.