/dev/disk1 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +250.8 GB disk1
Physical Store disk0s2
1: APFS Volume Macintosh HD - Data 149.4 GB disk1s1
2: APFS Volume Preboot 79.1 MB disk1s2
3: APFS Volume Recovery 527.0 MB disk1s3
$ mkdir /tmp/recv
$ sudo mount -t apfs /dev/disk1s3 /tmp/recv/
$ ll BaseSystem.dmg
-rw-r--r--@ 1 root wheel 496664671 Jun 18 15:09 BaseSystem.dmg
$ hdiutil mount BaseSystem.dmg
Checksumming Protective Master Boot Record (MBR : 0)…
Protective Master Boot Record (MBR :: verified CRC32 $D80226F2
Checksumming GPT Header (Primary GPT Header : 1)…
GPT Header (Primary GPT Header : 1): verified CRC32 $3488C747
Checksumming GPT Partition Data (Primary GPT Table : 2)…
GPT Partition Data (Primary GPT Tabl: verified CRC32 $FB2C82B9
Checksumming (Apple_Free : 3)…
(Apple_Free : 3): verified CRC32 $00000000
Checksumming disk image (Apple_HFS : 4)…
.............................................................................................................................................
disk image (Apple_HFS : 4): verified CRC32 $E3D25767
Checksumming (Apple_Free : 5)…
......................................................................................................................................................
(Apple_Free : 5): verified CRC32 $00000000
Checksumming GPT Partition Data (Backup GPT Table : 6)…
GPT Partition Data (Backup GPT Table: verified CRC32 $FB2C82B9
Checksumming GPT Header (Backup GPT Header : 7)…
GPT Header (Backup GPT Header : 7): verified CRC32 $B193AD93
verified CRC32 $C286C93C
/dev/disk2 GUID_partition_scheme
/dev/disk2s1 Apple_HFS /Volumes/macOS Base System
$ cd /Volumes/macOS\ Base\ System/
$ cd Install\ macOS\ Catalina\ Beta.app/Contents/Resources/
$ sudo ./createinstallmedia --volume /Volumes/test/
/Volumes/macOS Base System/Install macOS Catalina Beta.app does not appear to be a valid OS installer application.
Do I have to download full pack again, even when I just want BaseSystem to boot up?
createinstallmediacommand for macOS Mojave while substituting the appropriate application bundle name. Have a look at: How to create a bootable installer for macOS – user3439894 Jun 19 '19 at 22:52