How to list all products on my category pages as default?
it should display by default. Check if you have products and the catalog settings to display if it is set to 5 products change to a number which will display all your products. Also check how many products to be displayed per page and whether navigation controller is on ie. Next page menu.
On main page you can add a widget and set it to display all products
Use the below code in catalog_category_view.xml
<action method="setDefaultGridPerPage"> <argument name="limit" xsi:type="string">all</argument> </action>
This will show all products on category pages by default.