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I recently flashed LineageOS 18.1 to a OnePlus6T by following the official guide. It was successfull, but there are a couple of steps which I don't fully understand. The OP6T uses the A/B partition scheme and doesn't have a recovery partition like the older device I have used before; on that device I flashed TWRP to the recovery partition and it remained there persistently, allowing me to boot into it using a certain button combo. The guide to flashing the OP6T states that one must temporarily flash a custom recovery to the device using the command fastboot flash boot <recovery_filename>.img; My questions are as follows:

  1. Why is the flashing of the custom recovery only temporary? Is this because it will be flashed to the boot partition of the inactive slot (lets assume this is slot B) which is then loaded when the device reboots, only to be used to flash the actual ROM to slot A, which will then overwrite slot B the first time that it is updated?
  2. The command fastboot flash boot doesn't specify which A/B slot that the boot partition should belong to, does fastboot simply flash the recovery image to the inactive slot?

And one final question; the official guide contains the following line which I don't understand:

In some cases slot b can be unpopulated or contain much older firmware than slot a, leading to various issues including a potential hard-brick. We can ensure none of that will happen by copying the contents of slot a to slot b. This step is NOT optional.

Coming from a vanilla OEM install, why would one slot be unpopulated? Does this refer to scenarios in which the device has previously been used with a custom ROM which does not utilize the A/B partition scheme and may have left one empty? And finally, what does it mean by firmware? I have always thought that firmware was in general loaded from chips integrated into a devices mainboard/s and not something which is held in internal storage?

There's a lot to unpack there so feel free to elaborate or tell me that I have fundamentally misunderstood.

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    Note: the official site/guide of Lineage OS is actually on lineageos.org
    – Andrew T.
    Commented Aug 10, 2021 at 14:53
  • Smartphones are a technically a mixture between a PC and an embedded system. Therefore terms like "firmware" which is usually used for embedded systems is also used in the smartphone world. Also it sounds better to say "install a firmware" compared to "copy a system partition". Also "internal storage" is for most users the user partition where they have write access. The system partition (aka firmware) is by default read-only on Android.
    – Robert
    Commented Aug 10, 2021 at 16:53
  • @Robert I see, so in this context "firmware" is actually referring to the system files? I've only ever come across firmware in the context of PC motherboards and understood it to be the lowest level of software which is embedded into physical devices themselves and exposes some sort of interface to device drivers. To me the contents of a system partition (such as my Linux root filesystem) are something completely different to firmware... Commented Aug 10, 2021 at 18:07

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