Hello, Nataliyag2
Dashboard > Products > Categories or Tags > add new. Then associate them with products. Further assistance you can get from WooCommerce support.
“6) The related products on my site are random, they are not showing the same products that I add manually on WP on the product” – do you use Single Product Builder? If so, just show Up-Sells products – https://prnt.sc/Dd_xsAEiGRWp
Kind Regards,
8theme team