Mongo Memory management
No, MongoDB don't "read whole collection to memory" (automatically).
If you don't read/write collection B, then yes, MongoDB keeps collection A in the memory. 


So, basically MongoDB keeps (as long as there is enough memory to keep) all read and write data (indexes, documents) in the memory. Of course, you can "warm up" collection, by reading all documents. When memory is "full", the oldest documents are thrown away when newer data comes in.