How to find the Magento 2 backend's base URL?

You can find the backend’s URL suffix in the app/etc/env.php file:

The full Magento 2 backend’s base URL is <frontend base URL>/<backend suffix>.

You can find it faster with the command line.

Thanks for the solution.

Another way to check the admin URL is by running this command:

php bin/magento info:adminuri