As mentioned above, there are numerous ways of accomplishing this. For my codes, I use HttpService to make a get request to a Pastebin raw page of the codes in JSON format. I send a request to the page whenever a user redeems a code (with a debounce to prevent spamming) and therefore the codes are updated instantly.
I’ve found some other topics containing some of the other ways to accomplish this. This topic covers how to use the description of a Roblox asset to house your codes. It’s pretty simple and a good starting point if you don’t feel comfortable working with JSON and HttpService. The comments also offer some other ways to make this work.
This similar topic demonstrates a basic plugin that can be used to create and edit codes. It can prevent players from being able to use them more than once and is a great place to get started if you want it basic, but with lots of extra features.