how does the app affect Sound Quality once streamer connects to the provider endpoint?

Depends. You are correct that the user interface UI does not impact sound quality anymore than my TV's remote control affects movie quality, but Roon is a different matter.

Roon is a 3 layer (read: over-engineered) system with a UI, a core which handles all library and DSP functions and end points. In the case of Roon, the core touches all audio streams, there’s no way to avoid it. It does offer some neat features like a wide range of sample rate conversion (PCM to DSD, up/downsampling, MQA unfolding, etc.) as well as per device EQ.

As a result, the Roon UI has no sound but the Roon playback system might.

@ghdprentice  thank your responding.  you have adderssed the bits I already understand. 

OP (me) is questioning why many posters say aurender's or lumin's or or auralic's  roon app sounds better.  the SQ question- which should be moot when using anything but a pc/mac for a streamer... 
You would be a good one to debunk them all.  you have that swagger :)

you have seen this thread" The devil is in the details, those who have compared it, knows Innuos Sense and Aurender Conductor UI is much more faithful and natural sounding than ROON"  

maybe it is just a few who mistakenly attribute SQ to the control app?

I have a PS Audio Dac Jr with bridge2.  I don't have roon.  my only demo with the bridge is spotify connect (not hi-res)  my only streamer experience is the ML Unison which I bought at half price discount.  
as you are well aware, there are a lot of moving parts in a system, but my windoze pc has a lot more resources than any of the high dollar streamers.  how many have an intel i7 with 16GB or RAM?  zero.  so, it doesn't take that much CPU or flash to receive data and forward  to the DAC.  my laptop is idling to serve the bits

the Unison feeding the PSAudio seems to sound better, until I lower the volume to match the laptop source. then hard to discern.

still looking for my entry point into a streamer that makes the SQ difference.  I will need to demo it in my system Rogue Cronus Magnus III + Martin Logan ESL-X speakers.
 

Very complicated question if conflating methods.

Let’s start with the simple one. You own a streamer that does Qobuz natively. Like an Aurender, Auralic, or other high end streamer. In this case the streamer receives the music data directly to the streamer and you control what the streamer plays with an iPad or phone. So the iPhone or iPad is a “remote” to the streamer.

If you are using a PC, or iPad connected to Qobuz… all sorts of other options are possible. You could have the PC getting the music bits and sending them through a usb connection or through the air with Airplay to some DAC… this is a vastly inferior way of using the the service. PCs and Macs make very poor streamers.

The ballpark is leveled when devices (streamers) get Qobuz natively… and transmits to a DAC through a connection like SPDIF, or AES. In this case it is virtually always better than receiving the bits and retransmitting from a pc, Mac, or iPad. In this case typically a dedicated $20K streamer sounds better than a $10K streamer, which sounds better than a $5K streamer, which sounds better than a $2K streamer which sounds better than a $1K streamer, which sounds better than any PC, MAC or any other general purpose computing device.