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@gojushin The very short answer is: the curved areas in the base mesh need enough geometry to carry the edges of any intersecting shapes and their support loops.

If the overall shape of the existing stock model is acceptable then one of the easier solutions would be to apply 1-2x subdivision, cut out any slots or other surface features, clean up the topology around the shape intersections, and add another subdivision modifier to finish smoothing the final high poly surface.

Keeping things relatively simple and solving the topology flow issues around the shapes as part of the block out process will tend to make things a lot easier. It's also generally considered best practice to block out the larger forms first, adding mesh density when and where it's needed to support smaller details, so the existing edges in the curves can be matched to or used as support loops around the intersecting shapes.

Below is an example of what this process could look like.

Once the basic shapes are accurately modeled, move on to adding any intersecting shapes or cut outs and adjust the density of the base mesh so the existing edges can be used as support loops around the shape intersections.

Join in or cut out any intersecting shapes and clean up the mesh around the intersections to solve any major topology flow issues. Use the existing edges as support loops by landing the intersecting shapes between the segments of the curves and try to avoid causing any unintended mesh deformation when connecting edges and support loops to the existing geometry on the curved surfaces.

In the example below there's a few spots where some extraneous geometry is left in during the modeling process. Mainly to prevent other modeling operations from deforming the surface. This can be cleaned up, after the support loops are placed around the shapes, by merging down or dissolving any stray edges that would otherwise disrupt the segment spacing along the existing curves.

With these kinds of shape intersections on compound, complex curves it's important to: avoid unintentionally deforming the geometry in the existing curved surface, avoid bunching up support loops in a way that disrupts the segment spacing of the underlying curves, use the space between the support loops to make up for any differences between the intersecting shapes and the underlying curves, and try to keep the corner vertices / poles as close to the support loops as possible.

Here's what the final base mesh looks like, without and with subdivision smoothing enabled.

Though there are some exceptions that do have sharper corners, most inletted load bearing features on stocks tend to have softer or gradual shape transitions with radiused / fillited / rounded corners to prevent stress concentration. These types of softer corners are usually easier to model since they can typically be created using less geometry than sharper angled corners.

The same basic block out and topology routing strategies shown previously can also be used for modeling sharp or square corners on curved surfaces. It's just the base mesh may require more geometry to accurately hold both the curvature and sharper edges around the intersecting shapes.