If you visit any store in the EU selling new phones or tablets you will find only USB-C devices for sale. No enforcement required.

What the Chinese do with cheap products sold on AliExpress isn't important. Obliging big manufacturers to adopt common standards reduces environmental waste and helps cut down on gratuitous exploitation of consumers.

You can find plenty of 2nd hand devices for sale with non-USB-C connectors and no, the police, aren't arresting people for selling them, nor is their sale illegal.

Dumping things that aren't up to standard for sale in the EU is routine since the UK left the EU single market. The EU implemented full customs controls and goods inspections on day 1 after the UK left. To date the British still don't have either the infrastructure or the staff to do the same. As result it has become a magnet for substandard goods, both originating in the EU and passing through it (e.g. from Rotterdam).

The estimated cost for the UK to implement full controls amounts to more than the entire sum of the UK's near 50 year contributions to the EU budget. And that's only one of dozens of areas where the EU saves money by agreeing common standards.