What are the answers....what do they know??

with the Covenant trailers coming out it has my curiosity stirring. 

I have been watching all the movies from last to first (resurrection to Prometheus) and even watching deleted scenes trying to piece together what I can to figure out the answers to what I really want to know. 

My first question is Why? Why all this what is the ultimate goal of Weyland or perhaps Weyland/Yutani? 

in resurrection it is explained to the Ripley clone they want the DNA perhaps for weapons or vaccines it is something they believe could do so much. Ash even says in Alien they admire the organism. So they claim to explore to terraform and colonize but actually wish to bring back a specimen

I can see that but there is still pieces I do not understand and hope covenant can bring us closer. like, Who is sending out these transmissions and messages? in Prometheus, upon confronting the Engineer Weyland tells David "tell them we came just like they asked" So has Weyland been in contact before Dr Shaw & Holloway even made their discovery? I seems David knows more than we do; as if he is on his own mission even Vickers is not aware of and her agenda is even different than discussed with Shaw and Holloway as when Charlie asks her what is her agenda and Vickers responds that if they had the funds they would be on their own agenda so since they did not have the funds they are merely "employees" and not to engage with the Engineers if found by them. So Vickers already implies they may not be there for the same reasons....then we have David 8 doing his own recon and experiments even shutting of his video feed to Vickers as he combs through the Engineer ship. He "talks" to the still frozen Weyland on board and when asked by Vickers "what did he say?"  he only says "try harder" what does that mean? What does he know?? We find out that Weyland wants to live forever he envies David for not growing old and never dying and when asking the Engineer...meets his fate. So we do find out Weyland's plan but....how does he know of what resources the engineers have? Sure they believe they have the power to create and destroy (as seen in the opening of Prometheus) but how does Weyland really know and does he know more? And is that why David decides to trick Holloway into drinking the specimen? And knowing Shaw is carrying one in her? What exactly is David's mission and knowledge in all of this in Alien throughout it is always the Android who has a secret agenda (Ash, Bishop, David....even Call but hers is quite the opposite and another Topic) Bishop even says In Alien 3 "the company knew" they knew an Alien was on board with Hicks, Ripley and Newt (perhaps another part of the plan? we may never know... I think we need the Blomkamp Alien to help us but again perhaps that's another topic altogether)

And if David knows more how? How does he know this? 

And where are these transmissions coming from? 

Besides Prometheus....the distress call to the Nostromo...who sent it? Was it Engineers tricking more humans or perhaps Weyland corp knew there were eggs and facehuggers waiting and needed an excuse to send a group out so they could trick them into bringing one back? Ash knew like I said he said they admired the organism. So how did they know to go there?

 I hope Covenant gives us a little more pieces to put it together...my hopes are that Walter and David can answer more.