Question/Help Needed: Boomer who is totally inexperienced with DACs and streamers.

Since your Alexa device has a digital out  then you should be able to connect directly to the built in DAC of the NAD. From the link you sent, it looks like the DAC accepts USB in. If it does not have COAX in, then you may need to convert COAX to USB to get going. That would be a quick start to hear how the new amp sounds.  From there you can upgrade to a better dedicated streamer if and when you are ready. Dedicated streamers will sound better. But they use different software to control the music, which will take some getting used to. Tablets are usually better than from your phone. BlueSound is a logical step up and you can spend more on a streamer than you have in you entire system. It never ends. The BluOS software is easy and intuitive. Innuos software is very good as well. IMO, the control software is important. If you are not too tech savvy, then the software can be the Achilles heel of streaming enjoyment. Streaming services like Amazon, Qobuz, Tidal are supported by the BluOS. Amazon music may be more difficult to stream with other devices.

Yes, you can send a digital signal from the streamer (BlueSound or other) to the input in your built in DAC. And yes, control the streamer from a tablet or phone, and in the case of BlueSound, a PC as well.  And the BluSound Node has a built in DAC so you can try sending the signal from right and left RCA into the line inputs on your amp. Whichever DAC sounds better is your choice. As Soix stated, the Innuos line of steamers is very good and a step up or two from a stock Node. Music services like Qobuz will yield better sound on average since they offer high resolution files. What service(s) are you using for music now? If Amazon only, then the only streamer I know that is easy to play Amazon music is the Node. There are work arounds with other devices/software combinations, but more work and not necessarily plug and play like BluOS. I stream mostly from Qobuz for critical listening and then I have some internet radio favorites for background listening. I have Amazon as well and compared their HiRez and it was close to Qobuz but not quite there. But that was a while ago and it may be better now. Lots of "moving parts" to manage. My advice is take it slow, do one thing at a time, listen, enjoy, then consider upgrades. Too much at once can lead to frustration and diminish enjoyment. Upgrading the DAC down the road can improve the sound even further.

It looks like the BlueSound has optical and USB outs...so would I connect one of them to the corresponding input in the amp/DAC?

Connecting to your built in DAC with USB will support higher resolution files compared to Optical.