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I downloaded a ROM for a Xiaomi phone to see what's inside. It's a zip file, that has these inside:

boot.img                    META-INF               system.new.dat.br     vendor.patch.dat
compatibility.zip           product.new.dat.br     system.patch.dat      vendor.transfer.list
dynamic_partitions_op_list  product.patch.dat      system.transfer.list
firmware-update             product.transfer.list  vendor.new.dat.br

I understand that boot.img is what is loaded on boot. It has the kernel and a ramdisk. But how can the bootloaded, or the kernel, or the ramdisk access the other files like system.new.dat.br? Both system.new.dat.br and product.new.dat.br are 1gb large, so I guess they have the system.

So my question is: what are these files and how does the boot.img contents load them?

ps: .br is a compresses archive. It decompresses to .dat, but what are the contents of these?

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    Based on the file names I would assume that you don't have a full system firmware, instead you have a partial OTA update, hence those files only contain the changes that are applied to each partition on your device. How such a patch is applied can be seen e.g. in this tool: github.com/erfanoabdi/imgpatchtools
    – Robert
    Jan 5 at 8:36
  • Does this answer your question? Is it possible to flash a custom ROM without a custom recovery image
    – alecxs
    Jan 5 at 9:41
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    brotli --decompress --in system.new.dat.br --out system.new.dat; python sdat2img.py system.transfer.list system.new.dat system.img;
    – alecxs
    Jan 5 at 9:57
  • @Robert do you know a full ROM that I can take a look? I'm intersted in seeing how it looks fully
    – Gatonito
    Jan 5 at 17:45
  • @alecxs thanks, there's a folder images/ with lots of .img. Do you know where I can read more about it?
    – Gatonito
    Jan 6 at 1:47

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