Explanation
The read-only core file system in Adobe Commerce Cloud means that the core files of the Magento application cannot be modified. This significantly reduces the number of attackable surfaces on the website, making it more secure.

To add a custom router class to the list of routers in Magento, the routes.xml file is used. This file should be located in the etc directory of the module, under the appropriate area (either frontend or adminhtml). Within the routes.xml file, you define a router with an ID, a route with an ID and frontName, and specify the module that the route corresponds to. This setup allows Magento to recognize and utilize the custom router when processing URLs, directing requests to the appropriate controllers based on the custom routing logic defined.

If a developer fails to mention the start date in the "From" field when creating a price rule, the price rule will be saved. However, the price rule will not go into effect until the start date is added.
If a developer fails to mention the start date in the "From" field when creating a price rule in Adobe Commerce, the system will default to the current date, and the price rule will go into effect as soon as it is saved. The absence of a start date means there is no delay in the activation of the rule, and it becomes effective immediately upon saving.

Explanation
By default, third-party modules are located in vendor/ or app/code/ directories. The vendor/ directory contains modules that are installed using Composer, while the app/code/ directory contains modules that are manually copied or cloned from a repository.

Explanation
If a developer wants to allow the removal of columns from the database when deleting them fromdb.schema.xml, they need to define the removable columns in thedb_schema_whitelist.jsonfile. This file will tell Magento which columns can be removed from the database.