https://www.thegnomonworkshop.com/tutorials/designing-a-custom-kitbash-library

Schoolism -Kitbash libraries can be an efficient way to save time when producing quick concepts for a mood piece, creating objects for background set-dressing, or adding detail and the indication of functionality to hard-surface models. In this workshop by Gavin Manners, you’ll be taken through the process of how to design and build your very own hard-surface kitbash library.  Gavin’s workshop begins with how to spark the initial idea and write a brief for yourself. He walks through gathering references to be properly prepared for the creation of the assets in Blender. Also covered is how to get the most out of your kitbash parts by creating subassemblies, as well as the principle of exponential idea generation.  The workshop then details how to create mech designs based on the brief, explaining how to use your custom kitbash library as effectively as possible. A simple lighting and rendering setup is also demonstrated before moving on to the final paint-over process to produce a finished piece of concept art. Digital Painting