I assume accurized means to enhance the accuracy of the rifle. Who does this work? What do they do?
You are correct in your assumption about what accurizing means.
Gunsmiths (either amateur or professional) tighten the mechanics of the rifle and possibly replace the barrel with a better quality version.
These actions do not come without a consequence. Assault rifles are designed to function under varying conditions (sand, snow, mud, humidity, etc). They are intentionally designed to be "loose" so dirt does not affect their operation.
The accurizing of a semi-automatic rifle results in a much more finicky weapon that is prone to jamming.
I hope this answers your questions.