What you’re referring to are bangs, which Kagi has offered for a long time already. For your example that would be !g to search on Google. You can also add custom bangs.

The new snaps addition is something else: it gives you Kagi search results, but limited to that website. It’s the same as adding site:stackoverflow.com to your search query, but with an easier syntax (same syntax as bangs, but with an @ instead of an exclamation mark).