Holes from Textures

One easy way to create holes in 3D models, is by using a black and white texture. This is especially useful for cloth objects. However you can also turn it into geometry with thickness for 3D printing, with just a few modifiers.

The black parts indicate where the holes should be. This texture is then used as the Factor for a Mix Shader. It mixes a Transparent BSDF with a Principled BSDF node.

You can see the result in Material or Render preview.

The downside of this, is that the holes won't have any thickness. But that's less of an issue when using this method for holes in cloth objects.

If you want to 3D print the result, you can use the same texture in a Displace modifier instead. And then use a second object as a Boolean cutter. You can then use a Remesh modifier to recreate the geometry and a Decimate modifier to reduce the amount of triangles.

Then press Ctrl+A and choose Visual Geometry to Objects to create a duplicate object that has all the modifiers applied.