Basic DAC / Streamer questions

First of all, please be generous about old fool’s first step to the digital source.

I’m planning to switch to the digital source, and I’m looking for a product like Matrix Element X.
(It’s like Lumin X1 - DAC / STREAMER / Have its own Pre-Amp)

This DAC/Streamer seems to have pre-amp preinstalled. In this case,

1) If I have pre-amp, then is there any way I don’t use Matrix Element X’s pre-amp?
Or should every signal go through your pre-amp anyway?
(So Element X’s Pre-amp -> my Pre-amp? or once I plug this DAC to my Pre-amp, the sigal will bypass Element X’s Pre-amp?)

2) I plan to play songs from iPad with the app Flacbox. (Aiff files)
Without any subscription service, can I just use my airplay2?

3) Lastly, let’s say, my MacBook has its DAC and Maxtrix Element X has its DAC, and songs are played on MacBook.
This case how do I know which DAC is being used to make digital -> analogue?
(of course I want to solely use Element X’s DAC)

Thank you.
"literally performs almost same as..." 

You don't mention the basis for the comparison, whether a specific set of measurements or a subjective determination of sound qualities, but there are a lot of relatively inexpensive DACs on the market that many (but not everyone) love. For example, to mention only two, the Bifrost or Topping D90 DACs are under $1K and widely admired. Meanwhile there are other very pricey products that some have rejected as inadequate, or just overpriced.

Cost alone is rarely the sole factor to consider -- if they wish, a manufacturer could spend ten times more on the case than the electronic parts. Fancy cases make things impressively pretty but add little or nothing to the sound quality.  The trick is to listen to something before you buy or decide to keep it.

Thank you. I will contact them.

My thought that my questions are more general questions regarding DACs since these can be applied to every dac.

It is interesting that this DAC literally performs almost same as Mola Mola Dac that cost more than 10k.
The owners manual for this $3K device can be found on the manufacturer's web site.  On page 9 it explains that the line output can be set at a fixed level or the volume control can be used.

I'm not a Mac person, but the unit shows it has multiple coax and optical SPDIF inputs, a USB input and a IIS/LVDS input. So products that output a digital signal to one of those inputs can be used as a source.

The Matrix web site also shows an email address for support and also has a community forum. You should be able to get more specific information about your intended setup from them.