So in in home page we have FEATURED CATEGORIES section which show some categories with 
images and product count inside every category .

The bundle responsible of this section is OroCatalogBundle , Yes and start point layout is vendor/oro/commerce/src/Oro/Bundle/CatalogBundle/Resources/views/layouts/blank/oro_frontend_root/default.yml 

But the very important lines inside this .yml:

 
layout:
    actions:
        - '@setBlockTheme':
            themes: 'OroCatalogBundle:layouts:blank/oro_frontend_root/default.html.twig'
        - '@appendOption':
            id: page_main
            optionName: attr.class
            optionValue: ' offset-none'
        - '@addTree':
            items:
                featured_categories_container:
                    blockType: container
                    siblingId: featured_menu_container
                    options:
                        attr:
                            itemprop: 'hasOfferCatalog'
                            itemscope: ~
                            itemtype: 'http://schema.org/OfferCatalog'
                featured_categories:
                    blockType: embedded_list
                    options:
                        label: 'oro.catalog.featured_categories.label'
                        item_key: 'category'
                        items: '=data["featured_categories"].getAll([2,3,4,6,7,8,9,10])'
                        items_data:
                            categoryProductsCount: '=data["categories_products"].getCountByCategories([2,3,4,6,7,8,9,10])'
                        item_extra_class: 'embedded-list__item—hide-same'
​

Which defined the twig renderer file :

 
        - '@setBlockTheme':
            themes: 'OroCatalogBundle:layouts:blank/oro_frontend_root/default.html.twig'
​

 
                featured_categories_container:
                    blockType: container
                    siblingId: featured_menu_container
                    options:
                        attr:
                            itemprop: 'hasOfferCatalog'
                            itemscope: ~
                            itemtype: 'http://schema.org/OfferCatalog'
​

After that the block responsible of getting data and render it :

 
                featured_categories:
                    blockType: embedded_list
                    options:
                        label: 'oro.catalog.featured_categories.label'
                        item_key: 'category'
                        items: '=data["featured_categories"].getAll([2,3,4,6,7,8,9,10])'
                        items_data:
                            categoryProductsCount: '=data["categories_products"].getCountByCategories([2,3,4,6,7,8,9,10])'
                        item_extra_class: 'embedded-list__item—hide-same'
​

As you see :
items: '=data["featured_categories"].getAll([2,3,4,6,7,8,9,10])'
using layout data provider  -featured_categories- to call function  getAll with array argument of all featured categories , you can find the declaration of 2 data providers for featured categories and product count inside  : vendor/oro/commerce/src/Oro/Bundle/CatalogBundle/Resources/config/layout.yml :

 
   oro_catalog.layout.data_provider.featured_categories:
        class: 'Oro\Bundle\CatalogBundle\Layout\DataProvider\FeaturedCategoriesProvider'
        public: true
        arguments:
            - "@oro_catalog.provider.category_tree_provider"
            - "@security.token_storage"
            - '@oro_locale.helper.localization'
        calls:
            - [setCache, ['@oro_catalog.layout.data_provider.category.cache', 3600]]
        tags:
            - { name: layout.data_provider, alias: featured_categories }
  oro_catalog.layout.data_provider.featured_categories_products:
        class: 'Oro\Bundle\CatalogBundle\Layout\DataProvider\CategoriesProductsProvider'
        public: true
        arguments:
            - "@oro_catalog.repository.category"
            - "@oro_catalog.website_search.repository.product"
        calls:
            - [setCache, ['@oro_catalog.layout.data_provider.category.cache', 3600]]
        tags:
            - { name: layout.data_provider, alias: categories_products }
​

the 1 functions getAll([2,3,4,6,7,8,9,10]) and getCountByCategories([2,3,4,6,7,8,9,10]) is inside 
FeaturedCategoriesProvider ,  CategoriesProductsProvider .

Use Case

1 - Remove Featured Categories from Home Page :
inside you bundle of customization for example :
src/Ibnab/Bundle/CustomhomeBundle/Resources/views/layouts/blank/oro_frontend_root/default.yml
create default.yml and push :

 
layout:
    actions:
        - '@remove':
            id: featured_categories_container
​

 
    actions:
        - '@setOption':
            id: featured_categories
            optionName: items
            optionValue: '=data["featured_categories"].getAll([5,4,3,1])'
        - '@setOption':
            id: featured_categories
            optionName: items_data
            optionValue: 
                categoryProductsCount: '=data["categories_products"].getCountByCategories([5,4,3,1])'
​

3 - Other things  :
You can create dynamic categories by adding field in configuration section , and using inside your custom data layout  provider Instead data["featured_categories"].getAll([5,4,3,1]) use your data["my-featured_categories"].getAll([5,4,3,1]) , plz see how to create data layout provider .

You can customize the by adding src/Ibnab/Bundle/CustomhomeBundle/Resources/views/layouts/blank/oro_frontend_root/default.html.twig and fill with content of OroCatalogBundle:layouts:blank/oro_frontend_root/default.html.twig and start customization .