If I plug my phone into an external DAC, does it bypass the internal DAC in the phone?

Not if you use the headphone jack.
Otherwise, via Bluetooth or USB, yes.

E

iPhone? Or Android/other?

I'd like to know about an LG 30

Was hoping the OP would reply with iPhone or Android/Other before I replied. But........

Anyway, I have no idea for an Android phone. However, for an iPhone, I know you can get a bit-perfect digital output from the iPhone, without it going through the DAC. Apple pretty much controls everything iPhone and I believe that originally you had to use a product licensed by Apple to get bit-perfect output. One such device was the Nuforce Icon iDo. I have one of these and it works perfectly with both a late Gen-x iPod/iPhone the used the 30 pin cable. It also works with later Gen iPod/iPhones that use the Lightning cable. It takes a bit perfect digital stream from the iPod/iPhone and sends it through a box that have a volume control, DAC and analog outputs. It also has a coax and optical digital output that can go to a DAC and outputs the bit-perfect digital stream. Works great. 

I think you can now output bit-perfect digital from a late generation iPod/iPhone (w/lighting cable) using Apple's "Camera Kit". It's a lightning to USB cable that I believe outputs bit-perfect digital stream from the iPod/iPhone. 

I would recommend you go over to Computeraudiophile and do a search on Apple Camera Kit and see what they say. I think the camera kit will output bit-perfect digital from the iPhone.

Sorry, I don't know how to do it with Android.....

Reubent, thanks for information, but I’m
switching to android in a month so... yeah...

@jpierre - I highly recommend you go over an search the forums at computeraudiophile.com. If you can't find the answer, post the question over there. Good luck to you......

the lg v30 (which they're blowing out for cheap now) is as good-sounding a phone as you'll find--it outperforms a number of daps and headphone amps i've heard. only quirk is you either have to use higher impedance headphones (>50o) or trick the phone into high impedance mode with an adapter