Can I merge two datasets based on some field as key in Dataflow just using UI tools
You have to use the augment node in the dataflow to do this. Choose Left Source, Left Key (it is written in singular, but you can use multiple keys), Right Source and Right Key. Then check all right fields you want to add.
The field "Relationship" was a little puzzling to me at the beginning. Basically it just means, that all right fields get the prefix '{Relationship_value}.' in front of the Field name. if you don't want to have the fields preceeded by that, you can use a computeNode to just write the value to a new field without the relationship-prefix and finally slice off all unwanted fields.
