I don’t know why but my rig keeps moving my head along with the arm, there not connected, so I don’t know what I did wrong
Check in your weight painting view. Automatic weights can get things a bit wrong, it is making the best guess.
See if the head has any influence over it from the arm bone.
General Q&A note
Help us all to help you.
Please give full screenshots with any questions. With the relevant panels open.
Also, include the lecture time and name/number that is relevant to the problem/issue.
The screenshot can be done by Blender itself, via the ‘Window’ menu bar top left hand side.
On that menu drop down is ‘save screenshot’.
Close ups additionally where they help.
Along with what @NP5 has said, would you like to check for IK Chain Length in IK Constraints ?
I presume you might be having 3 chains instead on 2.