Millions of folks play Assassin's Creed because it's one of the biggest games in the industry. They play because few games transport you back to a specific time and place like Ubisoft's flagship series. They play because they like putting on a hood and stabbing guards with a hidden blade.
Others play because they're invested in the world of Assassin's Creed though. They care about the lore that Ubisoft has built up over several games, a few comics, some animated projects, and a big budget film. These people want to know more about the ongoing conflict between the Assassin Brotherhood and the Templar Order. They seek out new information about Juno, the rogue Ancient. They miss Desmond Miles and they know who Subject 16, Warren Vidic, and Juhani Otso Berg are.
This is for those players or those who are tentatively floating within that space. Here's many of the narrative connections between Assassin's Creed Origins and the rest of the franchise. Spoilers, of course.
Assassin's Creed II: Amunet
In Assassin's Creed II, Ezio eventually found a sanctuary underneath his Monteriggioni. Within this sanctuary were statues dedicated to the greatest Assassins from previous eras. By completing tombs related to each of the great assassins, you eventually unlocked the outfit of Assassin's Creed's first protagonist, Altair.
One of these statues was of an Assassin named Amunet. Amunet is depicted as a woman in Egyptian garb whose arms are entwined with a venomous snake. This is a reference to her greatest kill, that of Queen Cleopatra VII on August 12, 30 BCE. Amunet infiltrated Cleopatra's palace and killed her using an asp.
At the very end of Assassin's Creed Origins, upon the establishment of the Brotherhood, Bayek's Greek wife Aya cuts ties to her old life, fully becoming an assassin. When she does so, she takes the name of "Amunet", honoring her time in Egypt. During the game, Aya believes wholeheartedly in Cleopatra, only to be disillusioned when the re-crowned queen does not keep her promises. Aya threatens Cleopatra at the end of the game, telling her that she will die if she does not become the ruler Egypt deserves. You can guess how that goes.
So Aya is one of the greatest assassins in history by the time of Ezio. Bayek on the other hand, has never been mentioned until now. Brutal, bro.
Assassin's Creed (Film): Sophia Rikkin and The Animus Project
The Assassin's Creed film details the introduction of criminal Callum Lynch into the Assassin's Brotherhood. Lynch was kidnapped by Templar Order father-daughter duo Alan and Sophia Rikkin and forced to connect to his assassin ancestor Aguilar de Nerha. The events of the film end with Callum's escape from the Templar Order, Alan Rikkin's death, and Sophia vowing to kill Callum. (It probably won't be followed upon, as the film was not a financial success.)
A series of documents in Assassin's Creed Origins show that new modern day protagonist Layla Hassan was scouted for her Abstergo position by Sophia Rikkin herself. The emails also show Layla attempting to ingratiate herself in the Rikkin side of the company, offering advice on the physical, hydraulic version of the Animus found in the film.
Layla ultimately moves forward with her portable Animus project in order to impress Sophia. This portable Animus does not require the user to share DNA with the chosen target, only that some of that DNA is available. Layla uses her project to connect to the memories of Bayek and Aya, the progenitors of the Assassins.
Assassin's Creed: Revelations, Assassin's Creed III: William Miles
William Miles is the father of Desmond Miles, the modern day protagonist of all of the Assassin's Creed games up until Assassin's Creed III. He appears briefly in Assassin's Creed: Brotherhood, before taking up a larger role in the game's story as the leader of the broken and beaten Assassin Brotherhood. His part in the story of Assassin's Creed ended in Assassin's Creed III with the death of his son, Desmond. William left the Order for a time, leaving the organization to fellow assassin Gavin Banks.
Once the Templar Order took Desmond's body (FOR SCIENCE!), William re-entered the Brotherhood again, retaking his position as Mentor. His return to the fold and general activities are given greater detail in the now-defunct Assassin's Creed: Initiates community project.
William returns to the series towards the end of Assassin's Creed Origins, where he offers to let Abstergo Industries employee Layla Hassan continue her portable Animus project under the protection of the Brotherhood.
The Ancients: Isu Armor and The Order of Ancients
One of the additional outfits you can unlock in Assassin's Creed Origins is the Isu Armor. Given the name, the outfit hails from the First Civilization. They called themselves the Isu, while most of the characters in Assassin's Creed know of them as the Ancients.
The Isu created humankind and then the Pieces of Eden to control them. The antagonists of Assassin's Creed Origins, the Order of the Ancients, seek those Pieces of Eden and other Ancient technology to control humanity. Two of those Pieces of Eden, the Staff and the Apple, appear in the game. The Order of Ancients are the direct precursor to the Templar Order, though that connection is never fully made in Origins.
Watch Dogs: Olivier Garneau
Folks have joked before that Watch Dogs is actually the modern day Assassin's Creed, but Ubisoft has previously reinforced this idea with Easter Eggs. In Assassin's Creed: Black Flag, there were references to Chicago's CtOS, the huge surveillance apparatus that gives Watch Dogs' protagonist Aiden Pierce his "powers" via hacking.
In Assassin's Creed Origins, this connection is given even more weight. In the modern day sections of Black Flag, you play as an unnamed Abstergo Entertainment employee, working for CEO Olivier Garneau. Garneau leaves for a conference in Chicago, but is never seen again. On Layla's computer, there is an email showing a homicide that looks to be committed by Aiden Pierce. Layla surmises that the victim was Garneau.