Source material for digital files?

They are going to use what the record companies give them, whether it is new music that was recorded digitally or old material that was originally recorded to open reel. At some point, the record company has converted the tape to digital, and this may have been done more than once meaning there can be multiple digital versions running around.  

Of course, the same was true of analog. Many old recordings were multi-track open reel and mixed down to the two channel stereo format. However, there were often multiple pressing plants for vinyl records so there would be more than one copy of the master. In analog tapes, a copy of a copy represents a deterioration in sound quality. Even when you bought a LP, you never had any way to know which generation of the master tape was used -- the first or something further down the road. (This was also the same with vinyl pressings -- a stamper was only good for some many pressings before it had to be replaced. When you bought a LP at the store, you had no way to know if you got the first, highest quality pressing off of that stamper's run or if you got the very last one before they had to replace it.)  

At some point you just need to sit back and enjoy the music you like.

Well that is exactly my point: If the recording company converted the old analog file to digital, what format did they use? I think we can agree that if they used the CD format (16bit/44.1kHz) and the streaming services use that as the source for HiRes (I am simplifying here) then this would not produce the same result as using the old analog master and then digitize it with 24bit/192kHz.
Not sure what you mean with multiple digitizing? If they always used the analog master as source there should be no deterioration.

When a record company says they've given you a 16/44.1K or a 24/96K file, you pretty much just have to take their word for it. Yes, there are some tests you can run to see if there is any data in the file above 22K but even if there isn't, that's not a guarantee that they've misrepresented anything. There are lots of microphones that don't go much above 15K as well as a lot of tape machines that don't have much range over 20K.   I seem to recall there was a spate of misrepresented hi-rez material a few years ago but I think the companies have figured out there is more downside than up in playing that game.  

And, the same uncertainty about quality has always been true. Back in the old days when you bought a LP, you had no idea if it was from a first generation master or a fifth. Nor did they tell you if your pressing was from the first 10% of the pressings or the last portion of the run before they scrapped that stamper.  We would all prefer the better quality LP but never had a choice in the matter.  

As far as multiple digitizations of an analog recording, this is not uncommon with popular albums that have been reissued. For example, how many times have the Beatles albums been "remastered" for another release? In this process, they take the original tape and redigitize it, ostensibly to higher standards this time. And that often includes playing with the frequency response, applying additional noise reduction, playing with compression levels (especially if the reissue came at the height of the loudness wars) and so on.  (Also do not confuse remastering with remixing the multitrack studio tape which is much less common.) 

OP,

i think you’re over thinking the process. I would focus on what you can control, which is picking a decent streamer, DAC and preferably wired Ethernet connectivity. Once you do, you will find streaming quality from Qobuz and Tidal more than satisfactory and enjoyable.

In direct comparison, there are plenty of albums from 50’s and 60’s on Qobuz that sounds amazing when compared with vinyl.

https://www.qobuz.com/us-en/audio-quality