For security reasons, merchant requested to a developer to change default admin url to a...

The CLI command that the developer would use to update the admin url is magento-cloud variable:set ADMIN_URL. This command sets an environment variable called ADMIN_URL with a custom value for the admin url on a specific environment. Environment variables are configuration settings that affect the behavior of the Adobe Commerce Cloud application and services. By setting an environment variable for ADMIN_URL, the developer can change the default admin url to a unique url for every branch/environment of their Adobe Commerce Cloud project. Verified References: [Magento 2.4 DevDocs]