Debugging

The most powerful tool, and one Magento lacks out of the box. composer require --dev symfony/profiler-pack adds a toolbar at the bottom of every page in dev and a full request profile at /_profiler.

Profiler tab	What to look for
Doctrine	Number of queries, duplicates, N+1, EXPLAIN, mapping errors.
Events	Which listeners ran, in what order and how long they took.
Security	Current user, roles, firewall, voter decisions.
Performance	Request timeline: where the time went.
Logs, Exception	Logs of this request, the exception stack trace.
Validator	Validation errors per field.
Messenger, Cache, HTTP Client	Dispatched messages, cache hits and misses, outgoing requests.

dump($x) outputs to the profiler, dd($x) dumps and stops.
The profile of AJAX and API requests opens via the link in the X-Debug-Token-Link header.
In console commands, -vvv shows logs and the stack trace.
Xdebug works the same way as in Magento.