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.