You're looking for a useful paper or guide on how to use a flash tool for the Nokia G21. Here's some general information and a step-by-step guide on how to use a flash tool for the Nokia G21:
The primary tool used for the Nokia G21 is the SPD Upgrade Tool (also known as the Spreadtrum Upgrade Tool). This utility is specifically designed to communicate with Unisoc/SPD processors and flash .pac firmware files. Essential Preparation Checklist nokia g21 flash tool full