Honestly it's nothing new or something which is not available publicly. If you'll search on this same thread you'll find the same question been answered multiple times.
Get an entry point: In this case it's UART it's root shell. That's all, you've got root on a machine running linux(Android). Dumping, extracting, repacking are the easy parts Mapping and Reverse engineering the firmware is the one where either you run out of coffee or coffee run out of you.
I apologise but I can't write a step by step guide it'll take weeks also I have a day job and hope you understand I get nothing out of this . So I'll list out the points for anyone who is willing to learn.
1. TTL, UART - Basic level communication b/w machines- For establishing communication channel
2. Linux CLI basic file system commands will do- For firmware dumping, repacking - UBI reader
3. ARM assembly- For actual reverse engineering. Important files names - 'simlock', all files with prefix 'FX'
1. PC that can run IDA PRO and Ghidra - Mine's 9 year old - i3 2100 4GB RAM and I still love her
2. USB to TTL - Arduino will do.