How to use Weld

[micheus]

Weld is used to merge the objects, but they need to be close each other. In most cases we let they touching each other, once the regular command [LMB] uses a very short distance as tolerance. In some cases we can use [RMB] and set this value. This value will be related to the distance between the correlative vertices (the greater).

When the objects are far away as you are showing us one option would be to use Bridge - if there is no problem to create a new "segment" between the two objects. In this case, the action is made by selecting the two opposite faces (as you showed in the picture) and applying the Bridge command.

What is important remember for both cases is that these commands requires the two faces have the same number of vertices/edges.