So, in the new codex it points out that Guilliman "gives" the Deathwatch a legion of Primaris Marines. That seems to imply that they are not affiliated with any chapter--they are Deathwatch and always have been.
Is that accurate? Or is Guilliman merely levying specific Primaris Marines from various chapters?
If he *is* just giving them wholesale to the watch, how are they trained? What does their right pauldron look like? How's this all work?
There is a lot of information on how the Primaris marines were divvied up in the Dark Imperium book. Basically, a crazy large stockpile of Primaris Marines were built up by Cawl on mars from all the origional legions Gene Seed.
When Guilliman finished defending the Imperial Palace from an invasion of Khorne Daemons, he embarked on the Indomitous Crusade and brought all the Primaris Marines with him. This is a pre-horus heresy legion amount of marines, so many many thousand.
Durring the crusade, the Primaris Marines fought in squads of mixed legion heritage, but with no chapter iconography on their armour. This was a way for them to get used to their new equipment and such. As the crusade went along, Guilliman would give legions Primaris Marines of their gene seed stock, to replenish numbers, or create whole new legions to defend certain areas. So, he'd create a small chapter here, give out 30 marines there, give that chapter 40 marines etc, as they went along their way.
Makes total sense to me that the crusade went past a Deathwatch stronghold and Guilliman just was like "Okay, these 100 marines are now yours", dropped them off, and kept going.