Hello Everyone, I am trying to access custom development config from file src/config/Shared/config_d

Please have a look in vendor/spryker/config/src/Spryker/Shared/Config/Config.php which config gets loaded when

yes. if it exists. By default you should put everything in config_default.php

similar to

$config[MailConstants::SMTP_HOST] = getenv('SPRYKER_SMTP_HOST') ?: null;

no, same env-variable. the values would be set different by the deploy-script (the one which setup the services in the cloud etc). Are you in the spryker cloud?

yes, for local environment i guess - but you wrote about

if we have environment specific credentials like different keys for production and development environment

first rule: you should not store any credentials in code!

imagine you have two new settings:
my-third-party-service-user and my-third-party-password

you would set sth like in config_default.php

$config[MyConstants::MY_THIRD_PARTY_SERVICE_USER] = getenv('SPRYKER_MY_THIRD_PARTY_SERVICE_USER') ?: null;
$config[MyConstants::MY_THIRD_PARTY_SERVICE_PASSWORD] = getenv('SPRYKER_MY_THIRD_PARTY_SERVICE_PASSWORD') ?: null;

When you deploy to staging or production you can align with the dev-ops team to set the env-variables to the dedicated values while deploying the services.

This is the way to go for deployment but does not help you yet for you local environment.

For local development you have two possibilities:

A (clean way):
create config_local.php (IN ADDITION TO THE PREVIOUS CODE!) and put

$config[MyConstants::MY_THIRD_PARTY_SERVICE_USER] = 'my-user';
$config[MyConstants::MY_THIRD_PARTY_SERVICE_PASSWORD] = 'my-password';

B (will be committed, so not good for credentials):
set the default to your credentials rather then null in the config_default.php

$config[MyConstants::MY_THIRD_PARTY_SERVICE_USER] = getenv('SPRYKER_MY_THIRD_PARTY_SERVICE_USER') ?: 'my-user';
$config[MyConstants::MY_THIRD_PARTY_SERVICE_PASSWORD] = getenv('SPRYKER_MY_THIRD_PARTY_SERVICE_PASSWORD') ?: 'my-password';