One of the biggest questions I get asked from MCU fans is what is a Nexus being? What does that mean? Nexus beings are a well known thing in the comics and it looks like this idea is going to become more prevalent in the MCU so I am here to clarify what a Nexus being actually is, their purpose and at least in the comics, who they are.
First off Nexus beings are entities capable of changing the flow of time by affecting the future (reality warping, probability influence). They are essentially anchors of reality throughout the multiverse, allowing the multiverse to exist. The stability of the multiverse hinges on the existence of Nexus beings.
Theoretically no two Nexus beings can exist on the same plane (I say theoretically because they can project themselves into other realities, kind of like what we saw with Dream walking in Multiverse of Madness) and there is only one Nexus assigned to each reality. These beings, obviously very powerful, are the center, the connecting point of mystical energy in their reality (which is why it's so very funny to me when people try to claim that Doctor Strange is more powerful than Scarlet Witch).
It's also well established in the comics that Nexus beings give birth to absurdly powerful children, offspring almost always outgrowing the powers of their parents (for example; Wiccan, he has been proven to be way more powerful then Wanda, for once not out of sexism but because when a Nexus being gives birth they also pass on essentially an overload of pent up magical energy.
Now what we’ve heard in the MCU of Nexus beings is very limited; In Multiverse of Madness America says that she is a Nexus being and therefore has no variants and doesn’t dream, however that's not how being a Nexus works as a whole. Wanda is THE Nexus being for 616 and has variants, the very simple explanation for this is that America travels through the multiverse and therefore there can only be one of her, otherwise it would be chaos. That does not in any way eliminate other Nexus beings from having variants.
Each Nexus being also has a power unique to themselves that generally reflects the character of the universe that they are holding together, that said Wanda possesses nearly all of the special abilities possible as one of the only magic users in 616 to have access to every form of magic.
All of the known Nexus beings are; Wanda (616), Lore (literally just a Scarlet Witch counterpart, Eleyn (Witch Queen of Amazar, technically also earth 616 however several of these Nexus beings were short lived because they only existed as a part of Scarlet Witch’s story), Leonard Tippit (chosen by the Watcher to maintain balance his power was short lived, after he accomplished his goal he was taken down by none other than….Scarlet Witch. Check out the first 2 volumes of Avengers West Coast for more), Sise-Neg (Earth-74113, he is immensely powerful and pretty much literally created, or I guess recreated reality. This one is actually really cool so I recommend Marvel Premiere Vol 1 Issues #13-14), Merlin (The Nexus of the Otherworld and an intrical part of Earth-148, he also served as Sorcerer Supreme for a time. He has many more appearances than some of the other Nexus entities on this list, the biggest collected issues being Journey Into Mystery Vol 1, Doctor Strange and the Sorcerer Supreme Vol 1, Excalibur Vol 4.), Kang the Conqueror (Earth-6311, also named Nathaniel Richards he is said to have a distant relation to Reed Richards, or Doctor Doom so really he is tied to the Fantastic Four somehow. Kang has some stuff going on guys, two separate groups, the council of Kangs and the council of cross-time Kangs are real things that happened. So if time travel and Justice League crossovers are your jam then this is the Nexus being for you. Recommended comics; Avengers vol 1, Avengers: Forever Vol 1, Kang the Conqueror. Franklin Richards (Earth-772, Sue Storm’s second born child. Absurdly powerful but doesn’t really exist outside of the context of What If… stories.), Vision (Earth-90110 his story is almost identical to the Vision on Earth-616 and therefore only has a few appearances in the comics, also only exists in What If…), Odin Borson (Earth-9260, literally same situation as Vision), Jean Grey (Earth-9250, also only exists in What If…).
Hopefully this answers some questions, sorry it was such a long explanation I was trying to be comprehensive. Keep asking amazing questions, my entire existence thrives on answering comic book related questions!