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1. Developer restores
   Typically involves using a Nandroid, which is a complete or partial image of the eMMC sectors. Bit technical. Most partitions will require the device to be identical to the original. But sometimes sector state is too much hassle to backup/restore.
2. Consumer restores
   Typically file based, can restore apps and data. These are more likely to work across different models, but it won't be a full backup/clone if you care about restoring every memory sector. E.g. a "restored" app might be a newer version, or the partition sizes on your new device will be different, or the files may have different names, etc.

Remember, a full backup depends on what you want to recover from. There is no perfect solution at the 'consumer level' I've described. In EU/Asian countries, where people aren't so worried about denial of warranty, there is little reason not to install a custom recovery if it exists.

1. Developer restores
   Typically involves using a Nandroid, which is a complete or partial image of the eMMC sectors. Bit technical. Most partitions will require the device to be identical to the original. Sometimes sector state is too much hassle to backup/restore.
2. Consumer restores
   Typically file based, can restore apps and data. These are more likely to work across different models, but it won't be a full backup/clone if you care about restoring every memory sector. E.g. a "restored" app might be a newer version, or the partition sizes on your new device will be different, or the files may have different names, etc.

Remember, a full backup is not useful if it is too old, so you'll probably need to make regular backups. When chosing option 1 or 2 it is a tradeoff between how much you value the backup vs how much hassle you are willing to put up with.

A "full backup" seems to be a very overloaded term in the Android world. Allow me to first define it (paraphrasing a comment):

A full backup is anything that will restore your system state to when you made the backup. ~Izzy (OP)

So the obvious follow-up question should be: restore your state from what?

• Scenario 0: Your phone is physically broken.
• Scenario 1: Your entire drive (eMMC) has been blanked.
• Scenario 2: You can only boot into a USB flash mode like fastboot/ODIN.
• Scenario 3: You can boot into fastboot AND your (custom?) recovery
• Scenario 4: Your phone has been reset to factory mode. You have normal unrooted functionality.
• Scenario 5: Your accidentally deleted your favorite cat wallpaper image.
• Etc.

Each of these scenarios can have a different backup solution. A full backup would have to be able to address all of them. Of course I'm trolling a bit with the broken phone scenario (0), but it points out how ill-defined this canonical question is. This reflects the fact that many people and companies tend to oversimplify backing up and ignore the most important part: restoring.

Consider that in the Android community it is implied that hardbricks (0 & 1) are paperweights; you're expected to get a new phone. So let's divide the remaining backup solutions into 2 categories:

1. Developer restores (2+).
   Typically involves using a Nandroid (sector copy of most partitions) and maybe re-flashing a custom recovery. Bit technical. May affect the warranty.
2. Consumer restores (4+).
   Typically restores apps and data, including your cat photos. Partial restores could work across different models.

Both can be considered full backups for their use cases. Let the context determine if someone asking for a full backup wants option 1 or 2. There's a bit of gray area (3.5?) between scenario 3 and 4, e.g. if a user deleted files from the /system partition the restoration method may depend on what is missing.

Similarly, if you store your contacts in the cloud, there is usually no need to back them up. Just sync again after restoring.

A full backup seems to be a very overloaded term in the Android world. Allow me to first define it (paraphrasing a comment from OP):

A full backup is anything that will restore your system state to when you made the backup.

So the first obvious follow-up question is: restore your state from what?

• Scenario 1: Your phone is physically broken/lost and your replacement is a different model.
• Scenario 2: Your partition table was corrupted and now it won't boot.
• Scenario 3: Your entire drive (eMMC) has been blanked and it won't boot.
• Scenario 4: You can only boot into a USB flash mode like fastboot/ODIN.
• Scenario 5: You can only boot into your recovery mode.
• Scenario 6: Your phone has been reset to factory state.
• Scenario 7: You accidentally deleted your favorite cat wallpaper image.
• Etc.

The list points out how ill-defined this canonical question is. Each of these scenarios can have a different backup solution. A generic full backup would have to be able to address all of them.

The second follow-up question is: what is "system state"?

Are we talking about the conventional memory sector state, or the fuzzier user experienced state of functionality. Most users will really only care about restoring latter to some degree, in which case it may be more desirable to get an easier backup solution for the specific scenarios you care about. E.g. if all you care about is preserving your photos, a backup might be as simple as enabling automatic cloud storage in your photos app; a full backup would be a waste of effort/resources.

We can divide the full backup solutions (with bootable and possibly different phones) into 2 categories based on the definitions of state:

1. Developer restores
   Typically involves using a Nandroid, which is a complete or partial image of the eMMC sectors. Bit technical. Most partitions will require the device to be identical to the original. But sometimes sector state is too much hassle to backup/restore.
2. Consumer restores
   Typically file based, can restore apps and data. These are more likely to work across different models, but it won't be a full backup/clone if you care about restoring every memory sector. E.g. a "restored" app might be a newer version, or the partition sizes on your new device will be different, or the files may have different names, etc.

In summary, if you can restore the sector state, then functionality restoration should logically follow. But both can be considered full backups for their use cases. Let the context determine if someone asking for a full backup wants option 1 or 2.

Similarly, if you store your contacts in the cloud, there is usually no need to back them up again. Just sync again after restoring.

Though recently in Oreo there have been small improvements (XDA news) I still can't confirm if it can copy all user files like photos and music copied from your pc.