"Real" streamer vs. Bluetooth adapter--what's the advantage?

Hi folks. Please explain this to me in simple terms.

I’m currently streaming from a Galaxy 8 phone, Idagio and Amazon HD, to Audioengine B1 adapter connected by digital cable to a Hegel Rost integrated.

Suppose I got a ‘real’ streamer—Bluos or Cambridge or Marantz. Since I would be using a phone app to tell the streamer what to play, why would the sound quality improve? I’d still use the DAC in the amp, since I reckon that will be better than the one in a $1k or less streamer.

I’m pretty certain I’m missing something, but like the entanglement of sub-atomic particles, I just can’t get my head around it.

Any insight gratefully accepted.

Others are definitely more versed on this than I, but in short, there is a difference between using your phone as the streamer vs using your phone as the remote control device to control a ‘real’ streamer. In the latter case, the music is ‘originated’ by the streamer, not your phone. I’m not familiar with the B1, but it’s not possible to do hi-resolution streaming via Bluetooth, and I’m confident you would get a better sound via a streamer fed by either Ethernet or WiFi, while controlling said streamer by your phone via WiFi. 
I use Qobuz into a Bluesound Node 2i into a Hegel H95, and am thrilled with the results. I’m using the DAC in my Hegel, which I think is one generation newer than the DAC in your Hegel. With the Node 2i, it would be easy to use either the Hegel DAC or the Bluesound DAC, but I’m not any help in terms of passing judgement on either (I’ve never tried the DAC in my Node 2i). It’s not exactly Bluetooth, but I get a significantly better sound from Qobuz, via the BluOS app on my phone, via WiFi to the Node 2i, than I do from Spotify via Airplay to the Hegel. 
Hope that helps at least a bit. Here’s one seemingly relevant article: https://darko.audio/2020/11/no-bluetooth-cannot-deliver-hi-res-audio/

If you can’t tell the difference it doesn’t matter.
I'm getting a head ache.

Sorry - Spotify/Airplay isn’t a relevant comparison to Idagio/Amazon/Bluetooth. I shouldn’t have mentioned. 
Get a Bluesound node 2i.
it does Bluetooth, airplay and it’s own but I don’t know what it’s called.
see if you can tell the difference.
coys21’s description is accurate. Transmitting music with Bluetooth sends the music through the Bluetooth transmitter in the phone to the Bluetooth receiver that's part of your system. Bluetooth transmission compresses the signal from its original resolution, whether the music is stored on the phone or retrieved from a music service. There have been advances in Bluetooth quality but it started as a technology to connect keyboards and mice to computers and wasn’t originally designed for music.

When you instead use WiFi from the phone to a "real" streamer, the phone is just a remote control, as coys21 said. The phone tells a music service like Tidal, Amazon HD, etc., to send the music, uncompressed, through your modem and router directly to the receiver at the original resolution provided by the music service.
Since I would be using a phone app to tell the streamer what to play, why would the sound quality improve?

This is the same as using your TV remote to change channels or adjust the volume. The actual picture and sound quality is handled by the TV, the remote has no bearing on it. 

Get a Bluesound Node 2i, but make sure you DO NOT play via the bluetooth interface, or even Apple Airplay. Using direct ethernet or wifi  with the Bluesound remote app will give you the sound quality that will be substantially better than the other two options.
One of the beauties of the BS Node is that it allows so much experimentation on both the input and output sides. You can feed it by Bluetooth or Airplay, or direct.
I wouldn’t worry about using Airplay, if using it suits you. Airplay is lossless and serves up 44/16 red book - and nobody has been able to prove that higher rates or resolutions give audible differences.
Bluetooth on the Node is the aptX HD standard so if your phone is up to it this is “Hi Res” 24 bit. So actually “better” than Airplay (and standard CD) if you believe you can hear difference above 44/16.

Agree that Bluesound Node sounds better with Ethernet connection, made a noticeable difference with it in my bedroom system
I just got a Bluesound Node 2i as it has TIDAL select so I can use my Android as the controller through the cloud to the Node 2i.... I'll be using WiFi and from all that I researched and read users posts, there are no sonic advantages to an Ethernet connection vs. WiFi.
Bluetooth is a compressed format so sound quality would be better if you are feeding your DAC directly from a streamer.  Apple Air Play is uses your WiFi network and is not compressed.  I don't know if it is as good as direct streaming but it is much better than Bluetooth.  One of the advantages of Air Play is that some streaming services (e.g. Primephonic, which has an enormous classical catalog) use only Air Play or Bluetooth.
COY21's first post and Sfar's posts nail it.
Not all, but many other posts contain incorrect assumptions.
The Node 2i does all the streaming directly from the source via ethernet or wifi.
Your phone only serves as a full featured remote.
No, wifi and ethernet are not equal, but high quality wifi gear and a strong signal can be good.
No conversion to and from Bluetooth.
My phone and bluetooth receiver both support Aptx....it's good but it's still Bluetooth and not even close to what a hi res source can provide.

I'd wager that deleting the phone out of the formula will be a step up, although I still run a 'shop system' with the cell as the source.

I run Spotify on the main audio puter direct from the router sans the cell and I can't find anything to complain about....*S*   
I can recommend the Audiolab 6000n streamer, belting price of kit for the outlay, plays just about every source going. 
The explanation in my first post wasn't meant as a criticism of using Bluetooth if that's what's available to you or what you prefer, for whatever reason. I've helped friends put together systems using Bluetooth and Spotify as their main source and they're thrilled with the results. I use it in my car with long playlists on my iPhone and it sounds great. I was only trying to answer the question about the function of a phone in the system rsgottlieb described.