Well, I Can only hope I was nice! ;-)
>
> >
> >    I'm slightly confused by the question but before I get into that please provide the output of
> >
> > cat /proc/asound/cards
> $ cat /proc/asound/cards
>  0 [PCH            ]: HDA-Intel - HDA Intel PCH
>                       HDA Intel PCH at 0xf7340000 irq 137
>  1 [CODEC          ]: USB-Audio - USB Audio CODEC
>                       Burr-Brown from TI USB Audio CODEC at
> usb-0000:00:14.0-10, full speed
>
> > cat /proc/asound/modules
> $ cat /proc/asound/modules
> cat: /proc/asound/modules: No such file or directory
>
> (No wonder: I have it all in the kernel. But sound playing and
> recording are both fine, the problem is how to setup defaults that use
> two cards)

I don't think any problems are created by building the modules into the kernel. However I don't know if blacklisting (something I now think you won't have to do so it doesn't matter) a module works when it's built into the kernel works when it's built in.

> >
> > What desktop environment are you using?
> I use openbox (it is probably debatable whether it should be considered a DE)
> Anyway, this is a low-level setup, no higher-level stuff on top of ALSA.
> >
> > What kernel are you running?
> 5.6.4 (off portage)
> >

OK, looking at your problem I think you are asking for the audio input to come from the USB device and the audio output to be the built-in audio device. If that's correct then Alsa doesn't, by default, allow you to do that - input from one card, output to another card, but treating it all like a single card to make applications happy. What I _THINK_ you need to do is create a 'virtual' sound card. Google these terms

alsa virtual sound card

and you will find examples of how to do this. It requires editing .asoundrc. I've never done this myself so I probably cannot help much beyond this. (But I will try to be nice!) ;-)

As for my desktop question KDE, which I use, has something like this built in where it will create a virtual device that sends audio to all cards. I've used it (very briefly) sending audio to both USB and the built-in Intel audio device in my laptop, and it does work, but I eventually went a different way. 

As for my kernel question my Scarlett 2i2 3rd gen USB card didn't work correctly with kernels lower than 5.3. Seems like that shouldn't be a problem for you.

Wish I could offer more specific help on coding up .asoundrc but I suspect you'll figure it out pretty quickly.

Cood luck,

Mark