Huawei Nexus 6P :: OTA Update Download Location (On Device)?

I'm going to preface this with the nagging feeling I may be completely off-base with how I think OTA updates while rooted/custom recovery works, but here goes anyway.

I got my 6P at launch, and immediately went and rooted it. Everything's been fine except for the constant notification that there's an OTA update available. I've got TWRP right now, so just attempting to reboot through the system's standard update installation doesn't do anything.

I thought I'd just grab the manual updates and flash them from the web. I was positive I grabbed the right Before-to-After .zip, but trying to flash it through TWRP just results in it failing. Nothing breaks on the device, it just boots and runs normally, but I'd still like to get these updates installed.

What I'm currently thinking is that instead of chasing down build number update zips, I'd just use the downloaded one specifically and attempt to install it via TWRP. This poses the question:

1. Where does Marshmallow download OTA updates to when it attempts to flash them when you reboot?

and then the follow-ups:

2. Can I even do this via TWRP, or is there a different process that I should use that I've somehow totally missed?

3. Post-OTA update, will I need to re-root? (Not a big deal, just wanting to set my expectations)