If you use the new windows10.0-kb5000842-x64.msu file, it's combined and you don't need the ssu-19041.903-x64.cab, correct?
No, it is separate from the CU. There are good chances that you don't even need that update, but if it is offered on WU, then install it.
Is done by the 21H1 EP Afaik, is hololens specific, not applicable to normal SKUs Included in current CUs
Hmm so another 906 update, still errors out trying to apply it to the 19043.899 version, looking at the CBS it seems to be some certification signature file that is missing or something. "amd64_microsoft-windows-certificaterequesttool_31bf3856ad364e" Not too long until the usual monthly cumulative update though for April so it'll probably be resolved by then.
But it has a kind of order. All 21H1, 20H2, 19H2 are big mess. Look, You are expert in updates. I'm interesting about new features. Not updates. 21H1 is boring - it still doesn't have new backport visual feature.
That .899 LCU was a huge bag of hurt. I re-formatted my PCs and started over fresh to get rid of that junk. Here is my List of 19043.906 updates to apply over vanilla 19041.1 using W10UI_9.8 defender-dism-x64 SSU-19041.903-x64 Windows10.0-KB4589212-v2-x64_Intel_Microcode_Update Windows10.0-KB4601554-x64-NDP48 Windows10.0-KB5000736-x64_21H1_EP_844 Windows10.0-KB5000842-x64_LCU_906 Windows10.0-kb5000862-x64_DU_Sources_900 Windows10.0-kb5000983-x64_DU_For_SafeOS_899 Windows10.0-KB5001460-x64_FxP_678 Since I have Intel CPU, I use that Microcode update. AMD users can skip that.