Is it permissible to distribute ready-made design assets that have been collected over years from free and unknown sources, and is it permissible to give them to those who ask for them?

If you do not know that the owners of the materials prohibit their publication, uploading, or copying, then there is no blame upon you for publishing them or giving them to others for their benefit, provided that they do not contain anything forbidden, and that you do not give them to someone whom you know will use them for something forbidden.