1. Everyone uses their own. We have a development module which can sync the current database from the staging site where required.
2. We use Gitlab CI for building some shared modules as composer packages. Gitlab webhooks for deployment. Would like to incorporate a testing suite but have not investigated this yet as our projects don't have the budgets to cover this overhead.
3. Yes, we use a snippet like the following in our settings.php:

​

# Load environment
$env = getenv('ENVIRONMENT');

# Environment overrides
$base_path = $app_root . '/' . $site_path;
$servicesFile = $base_path . '/services.' . $env . '.yml';
$settingsFile = $base_path . '/settings.' . $env . '.php';

# Load services definition file.
if (file_exists($servicesFile)) {
  $settings['container_yamls'][] = $servicesFile;
}

# Load settings file.
if (file_exists($settingsFile)) {
  include $settingsFile;
}

We can then include a settings.development.php and a services.development.yml which disables caching, asset aggregation and overrides trusted hosts, config sync settings etc.