Assembly Definition + Packages = ?

An assembly definition file will cause all script on that folder level and below in the hierarchy to be compiled into a separate assembly. Think of it like a library. All your scripts that are not part of that assembly need an explicit assembly reference to that. I think, the editor already gives you an error messages that asks you, if you forgot an assembly reference. To create such a reference, all your scripts must be in an assembly with an assembly definition file, so that you can actually define references in the first place. Create an assembly definition file for all your scripts (e.g., directly in the assets folder, or in your scripts folder). That will encompass all scripts until a different assembly definition is found in the hierarchy below. Now, if you select your main assembly definition file, you can add references to other assemblies in the editor.

Some tripping stones:

• You cannot have cyclic reference. Scripts that depend on each other must be in the same assembly.

• References are transitive. That means if assembly A references assembly B, which in turn references assembly C, then you can use scripts of assembly C in assembly A.

• Editor scripts must be in a separate assembly that is marked as a test assembly (there is a checkbox in the editor for that). The same goes for tests (duh). This is because you don't want these scripts to be compiled in your final build. These test assemblies will need references to the assemblies of the respective runtime scripts, of course.

• When there are editor scripts in different editor folders that reference each other, you are forced to restructure these folders. I find the most viable solution to write a utility that will rearrange scripts and folders accordingly.

• When you have many external packages, many editor folders and a bunch of tests, you can end up with dozens of assemblies quickly. Here, I also suggest to restructure your folder structure.
  
  Personally, I think the concept of assembly definition files really needs improvement.
  
  Some references: https://docs.unity3d.com/Manual/ScriptCompilationAssemblyDefinitionFiles.html https://answers.unity.com/questions/1446378/assembly-definition-in-editor-folder.html https://forum.unity.com/threads/example-project-assembly-definition-files.482313/page-4#post-3296568

Problem: TMPro namespace is missing, when using TMPro;

Quick Solution: Click on your .asmdef file then in Assembly Definition References tab click on "+" and "add" the Unity.TextMeshPro from the prompt menu.

Explanation: Asmdefs and asmrefs are like chapters of a book. All the chapters need to be under the book so you can read it. You have to add the asmdef of TextMeshPro from the packages like this so your own asmdef will "link" to that codebase.