[Google Pixel 3a Questions & Answers] zimmie:
This was super helpful it gave me a good idea how the boot system is structured in Android.

It seems like "Citadel" is a code name used for the bootloader in the "OEM" Command settings.

There might me other code names under "OEM" command which is hidden deep in the code.

Already tried searching the Source.

"Fastboot reboot-bootloader" = reboots back in Fastboot mode.

"fastboot reboot-recovery" =

C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot reboot recovery
Rebooting into recovery FAILED (remote: 'Unsupported reboot option reboot-recovery')
fastboot: error: Command failed

"fastboot reboot recovery" =

C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot reboot recovery
Rebooting into recovery FAILED (remote: 'Unsupported reboot option reboot-recovery')
fastboot: error: Command failed

^ Now that interesting seems the recovery "kernel" is not available.

"fastboot reboot" = Cant find Valid operating system

Unfortunetly ADB was not enabled. I was too eager to flash a final version of 10 before doing any setup on the phone. The phone came with beta 10.

"fastboot getvar all" =

(bootloader) product:sargo
(bootloader) serialno:xxxxxxxxxx
(bootloader) variant:SDM eMMC
(bootloader) max-download-size:0x10000000
(bootloader) slot-suffixes:_a,_b
(bootloader) version-bootloader:b4s4-0.2-5402313
(bootloader) version-baseband:g670-00019-190426-B-5509456
(bootloader) secure-boot:PRODUCTION
(bootloader) secure:yes
(bootloader) hw-revision:MP1.0
(bootloader) is-userspace:no
(bootloader) slot-count:2
(bootloader) current-slot:a
(bootloader) unlocked:no
(bootloader) nos-production:yes
(bootloader) off-mode-charge:1
(bootloader) slot-retry-count:a:7
(bootloader) slot-unbootable:a:no
(bootloader) slot-successful:a:no
(bootloader) slot-retry-count:b:6
(bootloader) slot-unbootable:b:no
(bootloader) slot-successful:b:yes
(bootloader) battery-voltage:4241
(bootloader) battery-soc-ok:yes
(bootloader) partition-type:cdt:raw
(bootloader) partition-size:cdt:0x20C00
(bootloader) partition-type:xbl_a:raw
(bootloader) partition-size:xbl_a:0x380000
(bootloader) partition-type:xbl_b:raw
(bootloader) partition-size:xbl_b:0x380000
(bootloader) partition-type:xbl_config_a:raw
(bootloader) partition-size:xbl_config_a:0x20000
(bootloader) partition-type:xbl_config_b:raw
(bootloader) partition-size:xbl_config_b:0x20000
(bootloader) partition-type:tz_a:raw
(bootloader) partition-size:tz_a:0x200000
(bootloader) partition-type:tz_b:raw
(bootloader) partition-size:tz_b:0x200000
(bootloader) partition-type:aop_a:raw
(bootloader) partition-size:aop_a:0x80000
(bootloader) partition-type:aop_b:raw
(bootloader) partition-size:aop_b:0x80000
(bootloader) partition-type:hyp_a:raw
(bootloader) partition-size:hyp_a:0x80000
(bootloader) partition-type:hyp_b:raw
(bootloader) partition-size:hyp_b:0x80000
(bootloader) partition-type:fsg:raw
(bootloader) partition-size:fsg:0x200000
(bootloader) partition-type:boot_a:raw
(bootloader) partition-size:boot_a:0x4000000
(bootloader) partition-type:boot_b:raw
(bootloader) partition-size:boot_b:0x4000000
(bootloader) partition-type:keymaster_a:raw
(bootloader) partition-size:keymaster_a:0x80000
(bootloader) partition-type:keymaster_b:raw
(bootloader) partition-size:keymaster_b:0x80000
(bootloader) partition-type:cmnlib_a:raw
(bootloader) partition-size:cmnlib_a:0x80000
(bootloader) partition-type:cmnlib64_a:raw
(bootloader) partition-size:cmnlib64_a:0x80000
(bootloader) partition-type:cmnlib_b:raw
(bootloader) partition-size:cmnlib_b:0x80000
(bootloader) partition-type:cmnlib64_b:raw
(bootloader) partition-size:cmnlib64_b:0x80000
(bootloader) partition-type:modem_a:raw
(bootloader) partition-size:modem_a:0x6E00000
(bootloader) partition-type:modem_b:raw
(bootloader) partition-size:modem_b:0x6E00000
(bootloader) partition-type:msadp_a:raw
(bootloader) partition-size:msadp_a:0x40000
(bootloader) partition-type:msadp_b:raw
(bootloader) partition-size:msadp_b:0x40000
(bootloader) partition-type:reserved:raw
(bootloader) partition-size:reserved:0x3D80000
(bootloader) partition-type:abl_a:raw
(bootloader) partition-size:abl_a:0x200000
(bootloader) partition-type:abl_b:raw
(bootloader) partition-size:abl_b:0x200000
(bootloader) partition-type:dip:raw
(bootloader) partition-size:dip:0x100000
(bootloader) partition-type:devinfo:raw
(bootloader) partition-size:devinfo:0x1000
(bootloader) partition-type:apdp_a:raw
(bootloader) partition-size:apdp_a:0x40000
(bootloader) partition-type:apdp_b:raw
(bootloader) partition-size:apdp_b:0x40000
(bootloader) partition-type:spunvm:raw
(bootloader) partition-size:spunvm:0x800000
(bootloader) partition-type:dpo:raw
(bootloader) partition-size:dpo:0x1000
(bootloader) partition-type:splash:raw
(bootloader) partition-size:splash:0x20A4000
(bootloader) partition-type:limits:raw
(bootloader) partition-size:limits:0x1000
(bootloader) partition-type:toolsfv:raw
(bootloader) partition-size:toolsfv:0x100000
(bootloader) partition-type:logfs:raw
(bootloader) partition-size:logfs:0x800000
(bootloader) partition-type:ddr:raw
(bootloader) partition-size:ddr:0x100000
(bootloader) partition-type:sec:raw
(bootloader) partition-size:sec:0x4000
(bootloader) partition-type:bluetooth_a:raw
(bootloader) partition-size:bluetooth_a:0x100000
(bootloader) partition-type:bluetooth_b:raw
(bootloader) partition-size:bluetooth_b:0x100000
(bootloader) partition-type:fsc:raw
(bootloader) partition-size:fsc:0x20000
(bootloader) partition-type:ssd:raw
(bootloader) partition-size:ssd:0x2000
(bootloader) partition-type:dtbo_a:raw
(bootloader) partition-size:dtbo_a:0x807000
(bootloader) partition-type:dtbo_b:raw
(bootloader) partition-size:dtbo_b:0x807000
(bootloader) partition-type:modemst1:raw
(bootloader) partition-size:modemst1:0x200000
(bootloader) partition-type:modemst2:raw
(bootloader) partition-size:modemst2:0x200000
(bootloader) partition-type:persist:raw
(bootloader) partition-size:persist:0x2800000
(bootloader) partition-type:misc:raw
(bootloader) partition-size:misc:0x100000
(bootloader) partition-type:keystore:raw
(bootloader) partition-size:keystore:0x80000
(bootloader) partition-type:devcfg_a:raw
(bootloader) partition-size:devcfg_a:0x20000
(bootloader) partition-type:devcfg_b:raw
(bootloader) partition-size:devcfg_b:0x20000
(bootloader) partition-type:qupfw_a:raw
(bootloader) partition-size:qupfw_a:0x10000
(bootloader) partition-type:qupfw_b:raw
(bootloader) partition-size:qupfw_b:0x10000
(bootloader) partition-type:modemcal:raw
(bootloader) partition-size:modemcal:0x200000
(bootloader) partition-type:uefivar:raw
(bootloader) partition-size:uefivar:0x100000
(bootloader) partition-type:ImageFv:raw
(bootloader) partition-size:ImageFv:0x200000
(bootloader) partition-type:frp:raw
(bootloader) partition-size:frp:0x80000
(bootloader) partition-type:sti:raw
(bootloader) partition-size:sti:0x200000
(bootloader) partition-type:storsec_a:raw
(bootloader) partition-size:storsec_a:0x20000
(bootloader) partition-type:storsec_b:raw
(bootloader) partition-size:storsec_b:0x20000
(bootloader) partition-type:rawdump:raw
(bootloader) partition-size:rawdump:0x8000000
(bootloader) partition-type:vbmeta_a:raw
(bootloader) partition-size:vbmeta_a:0x10000
(bootloader) partition-type:vbmeta_b:raw
(bootloader) partition-size:vbmeta_b:0x10000
(bootloader) partition-type:klog:raw
(bootloader) partition-size:klog:0x400000
(bootloader) partition-type:metadata:raw
(bootloader) partition-size:metadata:0x1000000
(bootloader) partition-type:ffufw:raw
(bootloader) partition-size:ffufw:0x400000
(bootloader) partition-type:system_a:raw
(bootloader) partition-size:system_a:0xC2C00000
(bootloader) partition-type:system_b:raw
(bootloader) partition-size:system_b:0xC2C00000
(bootloader) partition-type:vendor_a:raw
(bootloader) partition-size:vendor_a:0x30000000
(bootloader) partition-type:vendor_b:raw
(bootloader) partition-size:vendor_b:0x30000000
(bootloader) partition-type:userdata:raw
(bootloader) partition-size:userdata:0xC7DB7BE00
all:
Finished. Total time: 0.634s

Thanks for the help.