Fixed vs Variable output

Standalone DACs with no volume knobs usually have a fixed output when you set their volumes to max in Windows. Which is usually around 2Vrms ? For example, To use Atom DAC+ and SMSL SU-1 DACs you have to set their volumes to max in Windows and then they both output around ~2Vrms ?

(By DAC/AMPs I mean combo units like: (Topping DX5 , Creative G6/X7 , FX-Audio DAC-X6 MKII)

With Some DAC/AMPs, The volume knob affects the RCA-outs and with some, it doesn’t. some even have a switch to change between fixed and variable output.
Is my understanding correct ? please confirm.

I have some questions:

Why do some DAC/AMPs have variable output through their RCA-outs that is controlled by the volume knob ? When using them as DACs only, why don’t they just output a fixed signal (~ 2Vrms) like a standalone DAC ? Does turning the volume all the way to zero makes the DAC/AMP to output a fixed signal ?

Doesn’t the signal get degraded when the output is variable ?

When used as DACs only(USB-Optical to RCA-outs), Do the following units output a fixed or variable signal:?

Creative X7
Creative G6
Zorloo ZuperDAC

They sometimes are used with active speakers or other suitable gear.
For speakers there needs to be some away to control the volume. The good speakers mainly are “full volume” so any signal they get, they blast of full volume.
So controlling volume from DAC/AMP (preamp) is one option.

Some speakers have volume control options. So from the speakers amp or DAC, you can lower volume. Same ways a DAC/AMP would do but one step before in the signal chain.

Depending of the quality or methods or options.
The signal might not “degrade” in a the sense it goes “bad”. Altered.
You might not notice a single thing cause its done in the “unaudible” level.
If you degrade the signal between more than one unit/device, you will eventually notice and hear differences. Depending how good the signal chain and processing is.