How to reinstall the system on an android phone completely?

Question

Android

Probably any "advanced user" knows that sometimes you want to "start from scratch". Get rid of the results of all experiments, traces of many hundreds of programs installed on trial and not completely removed programs (many of which, contrary to the general idealistic delusion, store their data and libraries not where they should be, but where they please), deep tweaks, which he himself had long forgotten about, but they affect something and can cause strange behavior.
Here's how to do it in the case of Android? Yes, so as not to damage the root is desirable.
By the way, when I finally (after more than a year of searching bit by bit (the smartphone model is very unpopular) and unsuccessful attempts, and naturally, I already forgot how I did it) rooted the phone with the help of, sort of, installing a specially modified Indonesian firmware, except for the appearance of SuperSu absolutely nothing has changed - hence I concluded that "flashing" ≠ "reinstalling the system".
PS: Unfortunately, neither CyanogenMod nor other alternative firmware are supported.
UPDATE: I would like the most understandable, detailed answers that do not imply any knowledge in this area at all. I'm not bad at desktop and server systems, but mobile is almost zero, so it's necessary "for dummies".

Answer 1

Every phone has a reset.
www.wikihow.com/Reset-Your-Android-Phone
You can also do a full wipe flashing via a bootloader like TWRP.

Answer 2

Most Android devices have access to fastboot mode.
Actually, the fastboot erase <partition_name> command will completely clear it. And the fastboot flash <section> <file> command will write a new image (it must be in img format).
PS not sure exactly if you need an open bootloader for this. But it is definitely not necessary to produce the firmware of the modem and the kernel in this way.