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If they are mechanical, they are probably some special sauce not quite USB-C ports. USB is digital, TRS is analog. (Yes that means standardized adapters have to include a DAC). The delay should be minimal, though.

There is a mode that's called "Audio Adapter Accessory Mode" that makes use of the USB 2.0 data lines and a few others as analog audio lines for the headphones.

That is good to hear! Hopefully smart phones are supporting that standard, because it does appear to be an extension of USB.

My understanding is that many USB-C adapters limit voltage, which screws over good headphones. Also from experience of using a DAC via USB-C everyday for many years, they're really really unreliable.

3.5mm on the other hand just works and is (was? ) ubiquitous.

There are passive and active dongles. With a passive dongle, the DAC is built into the phone and the analog signal is simply passed through the connector. Active dongles are essentialy tiny USB DACs, they communicate with the phone digitally via USB.