Faces can be extracted from a dumb solid using draft|offset (select face and T,R, escaping from the tool leaves a zero-thickness face) and extruded along edges, then added or subtracted from the body to move cutouts like those. Cylindrical and ellipsoid faces extracted with draft|offset can be part|...

I use TurboCAD Platinum and SharkCAD Pro, both use ACIS, and both have the branched loft function. You declare the trunk profiles first, then the successive branches, then finish. It's been in TurboCAD and PunchCAD/SharkCAD since 2007 or 2008, and it's trouble-free but not frequently used. Latest ve...

... add the individually lofted runners as the original workflow requires Where was that spelled out as a requirement? I suppose it could be argued that additive lofts to a part are concurrent single body operations, but there isn't a notional base that they're added to, IMO. Reminds me of Big Bang...

I tried GlouGlou's model, ACIS wouldn't shell/thicken it, citing internal boolean inconsistency. I tried another workflow, seemed brute-force, but whatever: lofting with the inlet profile circles initially offset, adding the lofts, then moving the inlet profiles to the intended location. I successfu...

That one doesn't have knife-edge splitters in the ports, if we're going to observe notions of flow management. The most recent version of ACIS refuses to add the individually lofted runners as the original workflow requires, too, but its branched lofting tool has been around for 11 or 12 years.

The duct I'm trying to make isn't exactly an organic shape, it's only a very basic aerodynamic consideration - to make airfoils smooth and continuous. It's still required to have very specific dimensions. Changing the dimensions or location of a profile enough to make the union work can be as minut...

Did you use the same workflow for FreeCAD and Fusion?

Yes, it wasn't spontaneous, untouched, it happens instantly when I select one of the branches and invoke offset. Just went back and tried it again, it offsets inside with a positive number, FC closes down when I declare a negative value. So I tried it again, offsetting internally from an outer skin ...

ACIS kernel used by many commercial CAD programs makes provision for this circumstance with branched lofting, the part is lofted all-of-a-piece at once. The result appears to be the same as you'd expect to result from adding the branches in FreeCAD. Shelling (offsetting) outwards is often more likel...

In part WB, you can try the offset tool with the space surface and offset filled. Offset is intended to give a consistent thickness, but when surfaces are contoured or not planar/cylindrical/conical/ellipsoid, offset may not work. If a workaround's needed, you can extrude surfaces instead of offsett...