Add to
.repo/local_manifests/a23.xml
<?xml version="1.0" encoding="UTF-8"?>
<manifest>
<project name="mrx7014/twrp_android_device_samsung_a23" path="device/samsung/a23" remote="github" revision="twrp_a23" />
</manifest>
Then run repo sync to check it out.
Kernel sources are available at: https://github.com/mrx7014/android_kernel_samsung_a23