Just FYI, in case you're not familiar, Unity is a cross-platform engine (like Godot, Unreal...even Ren'py) so in theory most of what you build and script is platform-independent and you can take a single project and tell it to make different builds for the supported platforms which in the case of Unity is web (HTML+WebGL+Javascript+WASM), Windows, macOS, Linux, Android, iOS... and thus you can make the most of your development effort. In practice however, there are platform differences like performance constraints for web, screen size, performance issues and touch input for mobile, etc. and you typically end up writing different code for different platforms distinguished by compiler defines
https://docs.unity3d.com/Manual/PlatformDependentCompilation.html
Also it's a lot of work and expense to test every single platform, so a developer might just focus on one or two.