Hello,
Navigate to Appearance > Menus.
Add your main categories and sub-categories as menu items >To make a sub-category, drag it slightly to the right under its parent category > Assign this menu to the main menu location.
WoodMart will render the submenu as a dropdown on hover for desktop.
For mobile, navigate to Appearance > Menus (same menu can be used).
Assign the menu to the Mobile Menu / Mobile Side Menu location.
If you have a large number of sub-categories, then you can use the Mega menu feature for this: https://xtemos.com/docs-topic/create-mega-menu/
Best Regards,