Does anyone know how gift cards and redeemable codes work?

I suppose this is a series of questions in one. I'm sorry about that. Basically what I want to know is what it would look like if I wanted to sell gift cards for memberships to my website.

Do companies with gift cards all come up with their own codes? Or do they hire some super security agency to produce the codes and then to validate the codes when they're submitted?

Thank you all in advance.

(Just some background information: I'm a linguist who runs an online platform. I'm trying to create something a bit more sophisticated, and this is something I'm considering for the platform. I'm here on Treehouse because I have to learn about programming and web development in order to get things moving. Here in Puerto Rico finding someone who will do anything digital for you for a reasonable price is impossible. Paying for a yearly subscription here will probably save me like $4,000.)