...what’s going on...someone explain it to me please

As far as ik Ryan and aoao are kinda reincarnation of the character they performed in drama. Ryan was a dragon Slayer and aoao was a powerful dragon , previous Ryan might had a twisted kind of love for previous aoao and led him to die. Kinda tried resurrect him again . And that thier story.
And the elder bro don't have any other personality but idk some incident happened leading to his memory loss . It may appear like double personality but he just forgotten all the bad deeds he done to glasses guy And the assault , not really clear but it might have in under the influence of sudden bloom (like heat) of the glasses guy.
I would suggest you read it again if
you have missed this many points.

Ok, let's see if this helps.

"do the big brother and Ryan both have two souls in one body?"

Ryan and the mc (AoAo) are the only ones that we know are reincarnated people so far, but saying they have "two souls" is actually this story's way of saying they each have one one soul containing two personalities (the current personality plus the personality of their previous life).

They just say it's like "two souls in one body" but keep in mind the Asian concept of soul is very different to the Western one. For many Asian religions, each individual's "spirit" can easily be made up of several "souls" as the same time (and not just by one, like in Western thought), where each "soul" would be representing just one aspect of the individual's entire "spirit's consciousness" (or whatever you want to call it).

We don't really know if Big bro is someone who was also reincarnated or not, we only know he did meet Xuan Yue (AoAo's past self) in the past, either when he was younger or in a previous lifetime of his own, and that he used to think of Xuan Yue as if he was his little brother, which has somehow become a real blood relationship now, since Xuan Yue somehow died and ended up being reincarnated as his current younger brother, Ao Ao.

"Why did the big brother assault the glasses guy?"

Big Bro still doesn't remember that he did that, so we haven't been told why. When he does remember, we'll probably be given a flashback to clarify why it happened. But you can already guess the big bro was under the influence of something else and clearly didn't do it of his own will.

"who is the guy in Ryan’s body and why does he want ao ao?"

He's Ryan's past self in his previous life, who was named Xiao Jifeng. Xiao Jifeng met and became obsessively inlove with AoAo's past self at the time, who was a dragon named Xuan Yue. Xiao Jifeng wanted to overpower Xuan Yue to make him his own, so he stole Xuan Yue's dragon pearl which containuedall of Xuan Yue's powers, in order to render Xuan Yue powerless, so that he could overpower him and make him his own. The legendary pearl AoAo is looking for is actually Xuan Yue's old pearl that is probably still inside the body of Xiao Jifeng's reincarnation (thus, inside Ryan's body) and the fact that everyone thinks that legendary pearl belonged to an "evil" dragon is probably Xiao Jifeng's fault, somehow.

I'm guessing the dragon pearl is inside Ryan's body.