...
PrestaShop's modules are most often found in the /modules folder, which is at the root of the PrestaShop main folder. This is true for both default modules (provided with PrestaShop) and 3rd-party modules that are subsequently installed.
Modules can also be part of a theme if they are really specific to it. In that case, they would be in the theme's own /modules folder, and there therefore under the following path: /themes/mytheme/modules
...