The Mach3 program controls CNC router from Changzhou Huajiang Electrical Co, model 6040 water-cooled.

Problem
When programming in Mach3 G-code, the placement of - for example X30 Y30 Z60 - "translates" into moving in cm instead of mm. Where should I look for the solution? Various YouTube videos and discussion platforms do not provide any clues or solutions.

Sounds like your Steps per Unit are out by a factor of 10.

On the Mach3 Settings page - run the Axis Calibration utility (Set Steps per Unit button) and follow the 'on-screen' directions to set each axis to the correct steps per unit. You will need to restart Mach after you have completed the process.

Tweakie.

Or just divide the numbers already in the steps per unit dros by 10.