Typically, to add your custom actions and filters you would add them to your themes or better still child-themes functions.php.
(Please see here why using a child theme for any customisation is by far the better and recommended way WordPress Child Themes)

However, if you want to keep all wppizza related filters and actions in another – perhaps more manageable place – you can also create a dedicated wppizza functions.php. This might be especially useful if you have also edited some template files directly and/or have added your custom.css in a dedicated file and have therefore already created a wppizza sub-directory of your (child)-theme.

So if you want to do that, do the following

• CREATE – if it does not exist yet – the following directory structure in your child theme (see notes below): /[abs-path-to-wp-content]/themes/[my-child-theme]/wppizza/
• COPY functions.php from /[abs-path-to-plugins]/wppizza/templates/functions.php to the directory we have created above, keeping the subdirectory structure intact. (you can also simply create a new functions.php file instead of copying)
• Add all your wppizza related functions to this file instead of the child-theme’s functions.php to keep them in a somewhat more organised place.
• Note: this file will be read AS WELL as other theme related functions.php. It’s simply an option offered for a bit more organisation in places

Although you could also use your main theme and create the above mentioned directory structure there, using a child theme is by far the better and recommended way to do this – see WordPress Child Themes