[FF14 Mobile]Class Showcase

[Lorika]

(Perhaps that's total nonsense though, I'm not a programmer. I just thought of Genshin/Infinity Nikki/Wuthering Waves/etc. who develop the same game for several platforms simultaneously. So I wonder do they really create every version from scratch or can you reduce the workload by using some code/programming across platforms?)

it depend. generally, since thoses games first target is the mobile market, they use either Vulkan or OpenGL as 2D/3D Render API.
OpenGL is the most cross-platform API, while Vulkan was dev for Android but it's based on an AMD technology (And AMD Chipset are used on X-box and PS4/5).

Or they use already well know and cross-platform Graphical Engine (like Unreal Engine, Unity etc...) and Game Engine to facilitate the port to another platform.

[Daralii]

it depend. generally, since thoses games first target is the mobile market, they use either Vulkan or OpenGL as 2D/3D Render API.
OpenGL is the most cross-platform API, while Vulkan was dev for Android but it's based on an AMD technology (And AMD Chipset are used on X-box and PS4/5).

Or they use already well know and cross-platform Graphical Engine (like Unreal Engine, Unity etc...) and Game Engine to facilitate the port to another platform.

Gachas have historically been built in Unity, though some companies have started moving to Unreal. I haven't had much luck looking up rendering APIs, but I think most at least use OpenGL and some version of DirectX.