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
Clone both this and the patches repos:
git clone https://github.com/Vgdn1942/lineage_build_unified.git -b lineage-19.1
cp lineage_build_unified/mk-treble .
Finally, start the build script - for example, to build for all supported archs:
./mk-treble -s
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.