Here is the process if you want to use your own mesh:

First, for the import and append ARkit part. the blend file location, names, mesh name and its collection matter.

the blend files have to be in the addon folder which is located at "C:\Users\YourUserName\AppData\Roaming\Blender Foundation\Blender\3.0\scripts\addons\RandomHeadGen".

the main head import blend file need to be named "RHG_HeadMeshFile.blend", inside the file, the head mesh that has shapekeys should be named "RHG_Head_Source" in "RHG_HeadMesh_Collection". while the ARKit blend file should be named "RHG_ARkit_Mesh.blend" and inside, the mesh that has the ARkit shapekeys is named "ARkit_HeadMesh" in "RHG_ARkit_HeadMesh" collection.

Second, for the main head, the script use prefix of shapekeys to determine which category it belongs. for shapekey that control General Head Shape, add "HS_" in the shapekey name. "CM_" for Common Feature. "FOREHEAD_" for forehead area, "EYE_" for eye area, "NOSE_" for nose area, "CHEEK_" for cheek area, "MOUTH_" for mouth area, "JAW_" for jaw area, "EAR_" for ear area, "FACE_" for face templates and "EMO_" for emotions.

Third, for hair particle systems. any hair particle system name start with "Male_" the Male Hair toggle button will switch its render as Path, while the toggle is off the render is set as None. same idea applies to hair systems with "Female_" prefix names.

Last for the material, the main head material should be named "RHG_HeadMat", and there are two mix node that control the blend, which are "Mix" and "Mix.001"

if you have done the above, the script will have no problem finding relevant target and execute. you can open the default blend files in the addon folder and check its structures.

Although you can use the default mesh to jump start the creative process, if you decide to bring your own, there are couple of free addon may be beneficial, Mesh Data Transfer which can transfer shapekeys between different meshes of different topology.  Transfer the Vertex Order, if two mesh of the same topology have different vertex orders, when join as shapes, the mesh would explode, transfer the vertex order fix that.