What are you asking, how to get the reference over?
If the thing(); that you’re calling isn’t static, you do need a reference to the object, otherwise how would the computer know which object you’re calling the thing(); on ?
So you could:
make it yourself (new it up)
receive it in a function call from someone else
put it in a field of something that you CAN drag in
Also, nobody is squinting at photographs of code in 2024. Use code tags please. That’s what they’re there for in the forum.
The mechanism used to include classes that do not inherit from the MonoBehaviour class is the same for the one that do. A class is a class.
In C#, your code is eventually compiled and “packed” in what we call an Assembly:
About C# Assemblies with Unity.
The mechanism used by Unity to package scripting units is actually a Microsoft standard called Assembly packaging.
An Assembly in C# or Visual C++ is the equivalent of a compiled binary output of a C or C++ program, once every translation unit produced an object file and got linked together.
On Windows, an Assembly can either take the form of a .exe, .msi or .dll file.
Although they have different extensions, their core structure is the same, giving interoperability between different software components.
The key difference is in the set of tools provided by the OS that targets Assemblies by kind. Thus even though all three kinds share a common structure, certain extensions are used for certain use cases, the most straight forward example being the use of .exe for software and .dll for dynamically loaded libraries.
But to be clear, they are exactly the same, it is possible to have some sort of “main” function (a system entry point) in all three kinds.
Additionally, static libraries with the format .lib are not considered Assemblies as they end up linked to an actual Assembly and do not produce their own.
Unity allows multiple Assemblies to be packages with a game.
Unity also prevents circular references between Assemblies. Thus if an Assembly A needs to call a function from an Assembly B, then B will not be able to call any API exposed by A.
For this reason, most of the code that could trigger circular references should be placed in only one main Assembly to prevent any design errors.
Then inside this main Assembly, namespaces should be used to properly draw the line between components.
It should not be done at the Assembly level (at least in Unity as of 2024, circular references are possible in standard dotnet, but Unity for now uses a custom integration of .NET that doesn’t allow it).
If there is code that is really not related to any Unity’s features and that can be dynamically loaded at runtime, it should be placed in an external self-contained Assembly that will go through a batch of unit tests before any deployment.
This can be done using the dotnet CLI.
For code that is non-external but self-contained with no possible circular references, it should be placed in a separate Assembly to avoid its re-compilation every time a change is made in the main Assembly.