Poor idle is likely caused by too low an idle speed, vacuum leak, sticking idle speed control valve, low fuel pressure supply eg blocked or restricted filter. Misfire.

Garage can sort for you.

Sounds like a bad Johnson rod.

I honestly don't know, but mine would do that announces changed oil it would stop

Took to repair, apparently was the purge hose.

Could try cleaning the carburetor? When was your last tune up? - Oil? Spark plugs?

When i start the car and rev it, it feels smooth but as soon as it idles it starts shaking then stops shaking then idles at 1k then drops down and shakes again

Holden Commodore berlina 2003