I need to make object with multiple materials, but some parts need to be "irregular" shape. I attached screenshot to explain better what I need to do. What is the best way to select part of object surface (similar to one on screenshot) so I can apply secondary material (mostly that would be some other color)
In the shader editor, create the first material you need. Then, duplicate the principled BSDF, and create the second material. Add a mix shader, and plug both BSDF into it.
In the factor, connect a mask texture. This is basically a black and white image telling Blender where to show each material. It can be made with a noise texture and a color ramp. Use CTRL + SHIFT + LMB to preview a specific node. This will help you to know what your mask looks like.
This is what the node setup could look like.
Of course, for the mask, you can use any black and white image you want, meaning you can also make it by hand in the texture painting panel. Hope this helps.
You can use masking in a node based material or select the faces you want to have the specific material and press "assign" on the material tab (make sure the object contain multiples material) here is a quick tutorial: https://www.youtube.com/watch?v=ms89wP8m4ZU&ab_channel=Olav3DTutorials
Here I used the knife tool to create the blue shape, it is not ideal as it could mess with the topology (there is a subdivision surface modifier on the object so the shape is more organic) so I would recommend using masking in node based texture.