Google Slides is a great tool for creating, sharing, and collaborating presentations. The presentation is stored in your Google Drive so it can be access anywhere, anytime. Google Slides does have a few templates for users to work from, but not many.
That is where sites like Slides Carnival come in. This site has many templates you can download to use with your Slides presentation.
Just download the slide deck, open the presentation in your google Slide account, go to the File menu and select make a Copy. You now have a copy to the document in your google drive and you will be able to edit, add, or delete slides as needed. The second slide in the template has a good explanation on how to use the template.