Here's a guide on how you can add products to your existing catalogs:
While in edit mode, navigate to the Catalog Editor.
Go to the product page where you want to add a product.
On the left side of the screen, click the "Product order" option.
It will reveal the Product Order menu.
As long as there is enough space in the current page, the "Add Products" button will be available. Click on it to start adding the product.
Select the product or variant you want to add to the page.
Click the "Add" button.
The new products will now appear on the page.