[How-to?] adjacent object meshes seamless, make invisible edges

Hello,

For several projects, I’m facing a recurring problem : how to make adjacent meshes seamless? How to make it a clean way? What are the good practices?
To give a few examples : EVE in Wall-E have seamless interlocks, or the doors for airplanes. For the colors and textures, there are ways for cheating, but making the edges invisibles, I wonder what is the best approach.

Post an example of what you have and what you want.

Typically, the problem appears in this kind of situation, when the purpose is to create something like a seamless door on a sphere.

It seems you tried creases to make unsubdivide edges… but for “inner edges” you have to use them “across” the edgse (or whatever you call this) pulled the door a little to see the difference:

Thanks a lot. The “invert” for crease edges was the part I was missing. I should have think about the “tension” constraint.

With auto smooth normals enable, even with solidify modifier, it looks quite nice.