It restores a decent amount of stuff but not everything. It's going to be getting substantially better in Android 11. The stock OS has slightly better integration of AOSP backup / restore because Play services has some extra bits and pieces to get more into app-based backups.
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They're gradually moving it into AOSP though, which seems to be what's happening in Android 11 to some extent.
github.com/stevesoltys/se
Every Android distribution does have LocalTransport as a sample / testing backup service implementation but it's just meant for the CTS to use.
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So, I don't think there's really anything else that knows how to use LocalTransport and there wouldn't be any guarantees of it not changing in the future. Essentially all these things back up the same data via the same AOSP API. adb backup does NOT use a backup service though.
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I think I could live with LocalTransport, I pretty much just want to have some way to restore most of my settings when wiping /data
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I'm not sure I understand how it works; do I set it through bmgr and have it dump everything to /sdcard or something like that?
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You'd need to set it as the active backup service but then I'm not sure how you would actually use it. I'm only familiar with it from running the CTS and watching it do stuff. No idea how to actually make it do a backup or where it puts it. Not sure how CTS fetches it either.
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android.googlesource.com/platform/cts/+ are the tests for that. It may be that it's really too annoying to use it. You're probably better off using Seedvault. It'd just be neat if there was an official local backup service instead of the legacy adb backup / restore.
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LineageOS is currently integrating Seedvault but they aren't already done. I can't quite use it but I also don't feel like redoing the work they're doing more or less right now
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I think I understand how LocalTransport works, let me see if I can get it to do something useful
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The general issue with system data is that the backup service is an app-based implementation, and there's important data outside of what's stored in apps. So, privileged system apps like Settings actually have to set up hooks that know how to backup / restore the information.
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I think what happened is that it's the Nexus / Pixel portion of the company that decided this was a problem, and they did a good job of solving it for the stock OS as a value add over AOSP via added backup hooks in their forks of apps and an extra app providing some backup hooks.
That's been slowly trickling into AOSP so it has been getting more complete. I'm not entirely sure how much stuff stored outside of app data is still missing backup hooks in AOSP. It's not actually very hard to implement at all, it's just hooks to load and save the data.
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So, apps have to go out of the way to disable it for themselves, but privileged system apps need to go out of the way to do backup/restore for the non-app-based data storage, like Wi-Fi networks and other things like that stored 'somewhere' outside of the system apps themselves.
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