How do I modify the contents of {{var alertGrid | raw}} in the template?
The full content of the mentioned variable is located in the following file:
/vendor/magento/module-product-alert/view/frontend/templates/email/stock.phtml
It is better to edit the mentioned file via the theme folder. For that, the file should be created in this path:
app/design/frontend/<VendorName>/<ThemeName>/Magento_ProductAlert/templates/email/stock.phtml
The created file can be used for editing the {{var alertGrid | raw}} variable.
If your theme is installed in the vendor folder, it won't be possible to redefine files via the app/design folder.