In Your Theme
If you want to override in your theme then create a following
directory structure
app/design/frontend/Vendor/Mytheme/Magento_Catalog/templates/product/list
Copy items.phtml file from below path
vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml
You can also override Layout file catalog_product_view.xml from vendor/magento/module-catalog/view/frontend/layout
to your
theme and make the changes you wish.
app/design/frontend/Vendor/Mytheme/Magento_Catalog/layout
<block class="Magento\Catalog\Block\Product\ProductList\Related" name="catalog.product.related" template="Magento_Catalog::product/list/items.phtml">
In your Module
If you want to override in your Module then create a following
directory structure
app/code/Vendor/Module/view/frontend/templates/product/list
Copy items.phtml file from below path
vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml
Override Layout file catalog_product_view.xml from vendor/magento/module-catalog/view/frontend/layout
to your
module and make the changes you wish.
app/code/Vendor/Module/view/frontend/layout
Remove Cache
php bin/magento cache:clean
php bin/magento cache:flush
Hope it Helps.