Can you charge a wireless mouse or headphones with a USB-C phone charger? Both devices came with a USB-C cable and are meant to be charged from a laptop.
I want to avoid warming, shortened battery life or any damage. I don't care however how fast it charges.
You absolutely can charge any USB device with a proper USB-C charger.
USB devices like mice, headphones, keyboards, and even phones can charge at the minimum voltage USB puts out (5V).
Power hungry devices like laptops that charge via USB-C may require more voltage to charge. Proper USB-C chargers can output more voltage if necessary (up to 40V or probably more now). USB-C chargers have electronics inside that will attempt to communicate with the device when it is plugged in. The device will let the charger know what its maximum voltage is, so that the charger will deliver it. If the device is old or higher voltage is not needed, it will not communicate with the charger and the charger will default to the minimum of 5V.
See these articles for more information: https://www.usb.org/usb-charger-pd https://www.howtogeek.com/769888/what-is-usb-power-delivery-usb-pd/
Yes.
There is no risk unless you get faulty* device. But we're talking here not just about cheap equipment, but straightforward scams.
If any device doesn't have special chip for power delivery negotiation, connection will work on default USB-C ratings (5 volts, up to 3 amperes). In practice it means that connection will be limited to 5 volt rating only and that receiver decides how many current to draw. And latter is defined by receivers circuit design and should be always safe for it.
*In unlike case of faulty charger design there is a risk of damaging receiver. Ie. if you get faulty charger which sends 9V by default. In case of faulty receiver, it will probably just not charge. Worse result would need it to be straightforward broken - ie. with electrical shorting.
