You have an issue with obtaining references, not with casting. Casting has nothing to do with finding references. You cast once you already have a reference. Focus on that part.
nobody talks about this menu. I have seen pictures of people using it in older versions of unreal, but when I try to select an asset, the menu disappears and nothing happens. What is going wrong here?
You’re seeing this dropdown here becase someone disconnected a pin that should not have been disconnected and the node defaults to a wildcard pin. Selecting anything there makes very little sense since objects generally do know about other objects’ existence - that’d be utter chaos and not the OOP way which BPs subscribe to.
What I was trying to do was to find a way to take any variable from any blueprint and put it in another blueprint. In many cases there would be no need to cast.
For this you need to know what those BP are, where they are, how they made it into the game and what logic you’re trying to achieve. There exists no one generic way of finding stuff - referencing and communication is circumstantial. No one method is better than others, some overlap in their utility and some are not suitable to certain tasks even though they can be made work.
If you have a concrete scenario / example and need advice, please do tell. You may have a situation where there is a doorBP and a buttonBP that opens it, both sitting in the scene - making them communicate is trivial and requires no casting whatsoever.
Some reading materials if you prefer to get to the bottom of it all on your own:
Actor Communication in Unreal Engine | Unreal Engine 5.5 Documentation | Epic Developer Community.
Check the links at the bottom of that page 
. And this is pretty good if you prefer someone talk at you in a soothing voice: