How to fill a gap between 2 objects? (different shapes)

[ivla]

Another method, one body without combining:
(working on a half-pipe, selection color is orange)

Select come faces:

Extrude it by X:

Flatten faces by X, dissolve all faces to one and move face as need by "absolute move":

Select and dissolve internal edges and vertices:


Select upper corner point and "intersect" along this edge to the lower vertice position:


Select and extrude by X face to form a base of the flange:

Select and extrude by normal side face(s) to form the flange:

Now you can set the flange size as you need - use absolute move and/or scale.

Cleanup "internal" face:


...and mirror on it, then weld a whole object:


Side faces on a "bridge" part will be non-flat so you can divide it by hand or leave this action for STL exporter.