Cut them up and use them for rags, or deface the logo with permanent marker before donation. Clothes that are not fashionable don’t sell. They end up in huge bales. A few are sold to clothing recyclers (cut up for industrial rags or shredded for felt), but most end up in countries where they disrupt the local garment economy, become landfill, or are burned, contributing to air pollution (do search for “clothing in Atacama desert” or “donated clothing in Africa”).
I only donate quality items in good condition that I would buy. Cheap clothing refills my rag bin. If you’re really feeling guilty about not donating used clothing, the best way to assuage your guilt is to become a resale store customer.