How do I blend the seam where two 3D components meet so the join does not look stuck on?

Why it happens

Combine modes join shapes by arithmetic. Merge keeps whichever shape is higher, which leaves a sharp valley where the two surfaces cross. Add stacks one height on the other, which leaves a step at the edge of the added shape. Neither one rounds the join. A soft, grown-together join has to be made afterward, on a single component, with the smoothing and sculpting tools.

Steps

1. Get the combine mode right first. The official guide's rule: a shape fully inside another is usually Add, and shapes that partly overlap are usually Merge, which gives a reasonable transition. A good starting join needs less sculpting.
2. Set the relative heights with Base Height, Tilt or Fade so the overlap is where you want it.
3. Make one component to work on. Sculpting more than one component forces a bake, and components on different Levels cannot simply be baked together. Use Create Component from Visible Model, which puts a copy of everything visible on the active Level. Then turn off the originals so only the copy shows.
4. Select the copy and open Sculpting.
5. Choose Smooth (key 1), set a Diameter a little wider than the seam and a low Strength, and stroke along the join. Smooth averages the high and low areas under the cursor.
6. Where one form should flow into the other, use Smudge (key 2) and drag from the higher shape into the lower one. It pulls material along like a thumb on clay.
7. Leave Preserve Transparency checked so the outer silhouette stays sharp (the 0 key toggles it).
8. Click Keep when a join looks right, then OK to finish.

Pro tips

• Under Mode, Raise only adds material and is good for filling the valley of a Merge join. Lower only removes and is good for knocking down a ridge.
• If you went too far, the Undo Brush paints an area back toward its original state.
• To bake a selection while keeping the originals, hold Ctrl when you click the Bake icon.
• For a gentle overall softening instead of local work, use Smooth Components (see that entry).