currently only Base S21 (o1s) and Galaxy S21 Ultra (p3s)

Create .repo/local_manifests/roomservice.xml and add this inside it:

<?xml version="1.0" encoding="UTF-8"?>
<manifest>
  <project path="device/samsung/o1s" name="Exy2100-LOS/android_device_samsung_o1s" revision="lineage-20"/>
  <project path="device/samsung/p3s" name="Exy2100-LOS/android_device_samsung_p3s" revision="lineage-20"/>
  <project path="device/samsung/universal2100-common" name="Exy2100-LOS/android_device_samsung_universal2100-common" revision="lineage-20"/>
  <project path="kernel/samsung/o1s" name="Exy2100-LOS/android_kernel_samsung_o1s" revision="lineage-20"/>
  <project path="kernel/samsung/p3s" name="Exy2100-LOS/android_kernel_samsung_p3s" revision="lineage-20"/>
  <project path="vendor/samsung/o1s" name="Exy2100-LOS/android_vendor_samsung_o1s" revision="lineage-20"/>
  <project path="vendor/samsung/p3s" name="Exy2100-LOS/android_vendor_samsung_p3s" revision="lineage-20"/>
  <project path="vendor/samsung/universal2100-common" name="Exy2100-LOS/android_vendor_samsung_universal2100-common" revision="lineage-20"/>
  <project path="vendor/firmware" name="Exy2100-LOS/proprietary_vendor_firmware" revision="master"/>
  <project path="hardware/samsung" name="LineageOS/android_hardware_samsung" revision="lineage-20"/>
  <project path="device/samsung_slsi/sepolicy" name="LineageOS/android_device_samsung_slsi_sepolicy" revision="lineage-20"/>
</manifest>

And then sync the repos by

repo sync --force-sync -j8