To add or create a Modifier (add-ons or options) to your items click on Stores or Menus, then click on Edit Menu.
Then click Edit on any item.
Click on Modifiers then choose to either Create a new group or Use/edit an existing modifier group.
You can name the modifier group and add a description for customers to easily identify the add-on/modifier.
The blue arrow is the MINIMUM number of options; setting this to '1' means they are required to choose. The pink is the MAXIMUM number of options to choose from.
In this case the MINIMUM is set to '0' where customers are not required to choose, but can pick up to 5 options for their toppings.
You can also create a Modifier create deals.
To upsell items you can create a Modifier to bundle it with an existing item.