How does a factory reset work?

Question

Android

There is a device. Beeline Smart is the first. As far as I understood this is nothing more than Huawei Ascend Y320
How does this procedure work in general? Technically?
It seems to me that somewhere there is an image of the firmware. With this procedure, all partitions are erased, then this image is written using the standard Linux dd utility. But is it?

Answer 1

A wipe cache of applications and phone data is made. The SD card is not touched. The kernel and firmware are not touched in principle. Therefore, after a rollback, in most cases, the root remains (because it is sewn into the core). On zenfon, for example, if you roll back, then the asus applications, launcher, modified curtain, etc. will remain as they were in terms of relevance at the time the firmware was created.