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

[ivla]

Another way to do it.
Filling the gap with "Bridge" operation.
Result will be clean if both bridging faces will have the same number of vertices.
Make pipe and flange, place it to the right position.
Make additional vertices on the flange face:

Turn on "wireframe mode" for the flange, select some face on the tube:

Dissolve it:

Select "unnecessary" vertices on face:

And delete it:

Then turn off wireframe mode and select two faces to be bridged:

Make "bridge".

Note that some faces on top and bottom requires a correction:

Use face dissolve and edge delete on top face:

and bottom one: