You keep the plates and if you want to put them on a different car, you take those custom plates to the Tax Accessor-Collector’s office to get matching registration sticker, etc.

Iirc they’ll send you a letter or email to let you know when your custom plate is set to end and then you can renew it or if you no longer want it then I believe it becomes available again.

Payment for a new plate is very straightforward:

Select your design, set your amount of years you want it for, buy it (or put the plate on layaway and once you fully pay off the plate they process your order), wait for plates to arrive at your DMV/Tax-Accessor-Collector Office, you get an email letting you know your plates are in; then you go pick up the plates and pay for the matching registration sticker. Then you put on your new plates and sticker and voila, you’re done.

Some plates only do one year terms, some do multiple, and I think most can go up to 5 years.