In short, you have three different processes running for various inputs and outputs. These namely are:
compiler to compile source code into object files.
meta object compiler to generate source code.
linker to link all the object files together into a binary, in this case shared librarry.
Step 1
The first created an object file out of your plugin source code using g++.
arm-linux-androideabi-g++ ... -o plugin_plugin.obj ..\plugin\plugin_plugin.cpp
Step 2
The second created an object file out of your myitem.cpp source code using g++.
arm-linux-androideabi-g++ ... -o myitem.obj ..\plugin\myitem.cpp
Step 3
The third generated the necessary moc file for your plugin using moc.
moc.exe ... ..\plugin\plugin_plugin.h -o moc_plugin_plugin.cpp
Step 4
The fourth created the object for the previously generated moc source code using g++.
arm-linux-androideabi-g++ ... -o moc_plugin_plugin.obj moc_plugin_plugin.cpp
Step 5
The fifth generated the moc source code for your myitem.cpp source code using moc.exe.
arm-linux-androideabi-g++ ... ..\plugin\myitem.h -o moc_myitem.cpp
Step 6
The sixth created the object file for the previouly created moc file using g++.
arm-linux-androideabi-g++ ... -o moc_myitem.obj moc_myitem.cpp
Step 7
The seventh and last step linked the created object file in the previous steps together using g++ for linking:
arm-linux-androideabi-g++ ... -shared -o libplugin.so plugin_plugin.obj myitem.obj
moc_plugin_plugin.obj moc_myitem.obj
These are all different steps working on different input and output files and then the file linkage happens for all of them. That is the reason why you got those commands run.