Lossless and Hi-res lossless

iPhone 6s supports Apple Music lossless either through EarPods connected via 3.5 mm jack or through internal speakers.

Smartphone internal speakers are obviously limited due to its size. For better results it's better to use external speakers.

Hi-res lossless (over 48 kHz) is available only with an external USB DAC (Digital Analogue Converter).

On iOS/iPadOS the only way I'm aware of to check ALAC/AAC playback is by looking at the icon under the playing progress bar. If it shows the lossless icon then it's playing ALAC, otherwise – without any icon – it's AAC.

Consider that currently 20 million songs are lossless. By the end of the year the whole Apple Music catalogue (over 75 million songs) will be covered.