What is the difference between a game engine, a rule set and the game as delivered?
A game engine is most often something that allows "people" to walk around (orientation and gravity) and fight (animation and hit boxes) in custom-made scenery. For example, that's why Star Citizen fails so hard on anything that isn't FPS.
A rule set can be a codex, but (in video games) most often it is a customized game engine that enforces all those rules. If done well, the boundaries and objects that use those rules are in text files that are read and used by the game engine. They have a fixed format.
In that sense, creating a game within such a customized game engine is mostly a matter of creating the 3D art and the text files that link those to the rules. Animation and interaction.
But, IRL, most game designers see that as too limited. They have Vision! So, to allow that, there is scripting as well.
Scripting is weird.
For starters, you would really want the developers to deliver something that can be compiled into native code. That makes everything so much better and faster. It also catches many bugs.
But, for that you need all the exceptions and interactions to be described strictly logical, in something that can be compiled, like a programming language. Which is seen as too restrictive by the artisans.
It requires the developers to learn a programming language, and especially to be strictly logical in everything. Which is often very hard for artistic people. They know it in their head and they can draw it, but animating it and seeing all interactions is too hard.
So, most game engines are shipped with a scripting language, that looks simple but allows endless fuckups. Because computers are strictly literal. They don't get "something like this". You have to make sure it all fits together into a coherent whole: not just the writing, but all of the mechanics as well.
And strangely enough, the best way to pull all that off is to make it clearly visible what is happening all the time. That also requires all the rules and scripting to be simple and understandable.
And it also has the added benefit that the player understands it as well.
Too often, the player navigates between nice scenery without understanding what is happening. Which might be cool for a hiking simulator, but is inexcusable for an RPG. Because those aren't about player reflexes, but about the reflex score of the avatar.