I suggest you learn about the difference between line level and speaker level. This article seems to do a decent job:

electronicshub.org/speaker-level-vs-line-level/

Your boiling water analogy does not fit - water boils at 100°C (depending on air pressure). It’s like the digital signal - boiled/not-boiled, on/off, 1/0, etc.

The output of a DAC (Digital to Analogue Converter) is a line level analogue signal and this signal has an amplitude (voltage) that can be controlled. I’m not a software or audio engineer so I don’t understand how, but my reading and own testing supports this.

My own simple test: I have a Google Pixel 4a and an Apple USB-C DAC (dongle). If I use headphones connected to either the phone audio jack or the DAC and any “normal” music player I can listen at full volume - it’s loud, but far from uncomfortable. If I use USB Audio Player PRO and configure direct hardware access to the DAC I cannot listen at full volume - it’s too loud.