These days, many recent smartphones no longer include analogue headphone jacks. Implicitly, the idea is to favor wireless Bluetooth headphones instead. As a result, if you want to hook up your wired headphones, one would need to buy a headphone adaptor which may come with your phone (for example, Apple at the beginning included their Lightning to 3.5mm Headphone Adaptor and these days may include some Lightning EarPods).
Recently, I upgraded my phone to the Huawei P30 Pro which does not have headphone output, so I was in the market to get one of these for the phone's USB Type-C digital connector.
Many of these headphone out / DACs are inexpensive, and the adaptor I have here today - the Soditer USB Type-C Headphone Adaptor - ~US$25, is advertised as supporting hi-res playback (even up to 32/384), and is claimed to provide up to 35mW into 32Ω headphones. Let's have a look...