The Samsung GT-C3520 is a classic feature phone (part of the "Champ" series) running on a proprietary Samsung operating system, not Android. Over time, these devices can suffer from software issues like boot loops, hanging on the logo, forgotten passwords, or IMEI null problems. The solution is often to "flash" the device using a specific and a compatible flashing tool .
Flash Loader 7.5.4 CSC v0.2 Citrus Lite (Standard tool for this model). samsung gtc3520 flash file and tool
Extract the Flash Loader folder and run the bfloader.exe file. The Samsung GT-C3520 is a classic feature phone