This project aims to help you build RattlesnakeOS, an AOSP ROM targeting the (Google) devices actually supported by AOSP, from Android 9 Pie onwards, locally, without using the AWS stack.
It's a mainly privacy/security focused ROM (no Google Play Services neither root by default).
Regarding this project name, and since we all like having a fully working smartphone with push notifications and so, it's also possible to bake in some patches before building the ROM.
At the end, you'll get a flashable ROM customized to your need.

• pure AOSP
• locally built (no proprietary cloud used)
• signed with your keys (only you can push ROM updates to your phone)
• bootloader is relocked after first install: no rogue 'fastboot boot/fastboot flash' commands can be issued to your device
• secure boot aka Android Verified Boot
• F-Droid and F-Droid Privileged Extension to allow easy installation of FOSS apps
• Optional: OpenGapps (while still retaining locked bootloader)
• Optional: Magisk (while still retaining locked bootloader)
• Optional: misc. patches (custom bootanimation, add/delete entries in recovery menu, add new permission toggles and so)

The ROM will be built on your computer/server so, the prerequisites of AOSP has to be met: Establishing a Build Environment
Keep in mind that AOSP and Chromium will be built in the process, so the whole build will take many hours.
I'm personally building this on a (quite powerful) computer (4c/8t Core i7, 32GB RAM, 1 TB SSD NVMe) with Ubuntu 18.04 and the whole thing is compiled under 5 hours (maybe less, can't remember)

./prerequisites.sh
./build.sh [-m] DEVICE FORCE_BUILD AOSP_BUILD AOSP_BRANCH

The -m argument will build Magisk in.

DEVICE --> The codename of the device you would like to build, e.g., sargo for Pixel 3a FORCE --> Whether or not you want to force a re-build, even when an updated version isn't available. You should probably use false so as to avoid unnecessary rebuilds AOSP_BUILD --> The android build ID, as listed on https://developers.google.com/android/ota. For example: qq2a.200405.005, for the April 2020 update AOSP_BRANCH --> The source code tag, as listed on https://android.googlesource.com/platform/manifest/+refs. For example: android-10.0.0_r33, for the qq2a.200405.005 build

Putting it all together:

./build.sh sargo false qq2a.200405.005 android-10.0.0_r33

Once the build is done you'll find yourself with flashable zip files within $CHAOSP_DIR/out/release-$DEVICE-$BUILD_NUMBER/
You can now follow the same guide than RattlesnakeOS: Flashing guide

• add a -g argument to the script to toggle or not, the integration of OpenGapps (actually, pico package is always built-in)
• add an option in recovery menu to delete all Magisk-related settings/modules to avoid a lock-out situation (when bootloop occured, etc.)
• replace Chromium with Bromite as a Browser/WebView
• add an option to use microG Project instead of proprietary Google Play Services (when using OpenGapps)
• find a way to build Magisk during the building of CHAOSP instead of downloading Magisk releases zip files from GitHub

• @thestinger for his work on the now deceased CopperheadOS, and newly started GrapheneOS
• @dan-v for his work on RattlesnakeOS from which near the integrity of the build Go template is re-used here
• @topjohnwu for the only FOSS Android rooting solution: Magisk
• @anestisb for his handy script allowing us to retrieve important missing and non-git commited binary blobs for our devices : android-prepare-vendor
• @PabloCastellano for his handy DTB extracter script: extract-dtb
• OpenGapps for their aosp_build project
• many different people for the useful FOSS marketplace: F-Droid