Multi environment-lite
Alright, so turns out all I need to do is comment out the following line in index.php:

define('CRAFT_ENVIRONMENT', getenv('ENVIRONMENT') ?: 'production');


Now I can use config.php keys, same way I did in Craft 2. For example:

'*' =&gt; [
    // My default/production variables
    'devMode' =&gt; false,
    etc...
],
// Local
'address.test' =&gt; [
    // Local variables
    'devMode' =&gt; true,
    etc...
]
// Staging
'staging.address.com' =&gt; [
    // Staging variables
    'devMode' =&gt; true,
    etc...
]


Thanks to Luke James for the assist on Slack.