The Drop Down input widget can only be used for attributes that are enumerations. However, you can use the Reference Selector input widget to set associations between objects. So if you want to use an attribute value of another entity to populate a dropdown, simply make a many-to-one association between the two entities and put a reference selector on the page.
I don’t fully understand the second part of this question. If you want to put a drop down or reference selector, you will need to put it in the context of an entity, so it will need to be inside a Data View. Can you be a little more detailed about your use case?