To get started with Android/LineageOS, you'll need to get familiar with Source Control Tools.
To initialize your local repository using the LineageOS trees, use a command like this:
repo init -u https://github.com/LineageOS-R-ASB/manifest.git -b lineage-18.1 --git-lfs
Then to sync up:
repo sync -j$(nproc)
Btw, if you already have 18.1 synced, you don't have to delete it and fully resync this one. Just run the init command on top of your 18.1 buildsystem and it'll automatically fetch the modified repositories :)
The backports are taken from LineageOS Review (R_asb_20XX_XX tags)
See build repository to check what security patch is merged.