To get started with building LineageOS GSI, you'll need to get familiar with Git and Repo, and set up your environment by referring to LineageOS Wiki (mainly "Install the build packages") and How to build a GSI.
First, open a new Terminal window, which defaults to your home directory. Create a new working directory for your LineageOS build and navigate to it:
mkdir lineage-19.x-build-gsi; cd lineage-19.x-build-gsi
Initialize your LineageOS workspace:
repo init -u https://github.com/LineageOS/android.git -b lineage-19.1
Clone both this and the patches repos:
git clone https://github.com/AndyCGYan/lineage_build_unified lineage_build_unified -b lineage-19.1
git clone https://github.com/AndyCGYan/lineage_patches_unified lineage_patches_unified -b lineage-19.1
Finally, start the build script - for example, to build for all supported archs:
bash lineage_build_unified/buildbot_unified.sh treble A64B A64BG 64B 64BG
Be sure to update the cloned repos from time to time!
Note: VNDKLite and Secure targets are generated from built images instead of source-built - refer to sas-creator.
This script is also used to make device-specific and/or personal builds. To do so, understand the script, and try the device and personal keywords.