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@rudenko_je Welcome to Polycount. Consider checking out the forum information and introduction thread.

This thread has a lot of great resources so it's probably worth taking some time to skim through the recent posts and look for examples of how other artists have solved similar problems on other shapes.

A couple of posts up there's a recent write up that covers the basics of cutting holes in curved shapes without creating noticeable smoothing artifacts. The same basic concepts can be applied to these rectangular slots on the side of the cylinder.

A few pages back there's also a couple of other posts that cover different approaches to similar topology issues.

Provided there's enough geometry density, the new edges that run parallel between the existing segments of the cylinder shouldn't cause any major smoothing issues. If the new edges do cause minor smoothing issues then it may be necessary to slide them along the surface of the cylinder to relive the smoothing tension caused by the subdivision.

Another strategy would be to use the existing geometry of the cylinder as part of the outer support loop around the shape intersection. In most cases this can be achieved by offsetting the intersecting geometry or using inset operations to create the inside profiles of the cut outs. When joining or subtracting shapes from curved surfaces it's important to match the segments of the intersecting shapes and provide some space between the existing geometry for the support loops to take up any difference between the geometry that defines each surface feature. This topic is covered in depth a few posts up in the thread.

Here's an example of what this process could look like when cutting a rectangular shape out of a larger cylinder. Start by adding the support loops on either side of the intersecting shape. Select the interior faces that define the area of the rectangular cut out and run an inset operation. Dissolve the interior geometry to create the rectangular cut out. Add the perpendicular support loops near the corner of the shape intersection then add the support loops on the inside of the rectangular cut out.

Either approach will work, when using the appropriate amount of geometry, on both solid and hollow shapes.

Provide a clear route for support loops to run around the shape and terminate unnecessary support loops in an area that won't disrupt the underlying topology. When merging shapes with curved surfaces it's generally going to be a good idea to match the segments and provide some space between the existing geometry elements to act as a support loop and take up any difference between the shapes.