CD vs FLAC stored on flash drive vs streaming

As usual, depends. The better your CD playback equip. is, the better it will sound. A decent streamer and a great DAC will sound better than most CDP’s <$1K.
Great streamer usually doesn’t sound as good as a great CDT into the same DAC.
Absolute questions around here are deficient.

The only general "rule" in audio is that if it sounds better it is. Knowing what "should" sound better to you doesn’t mean it will.

That's generally what I've heard. My assumption is that the digital file via a CD player or via a flash drive has a shorter signal path to traverse and thus less chance to pick up analog noise along the way compared to the longer signal path that the streaming signal takes. Noise might include common-mode noise such as RF interference or  and differential-mode noise as typically caused by ground currents.

I have come to accept, "sounds good enough to be enjoyable" as my end point.  I cannot reliably determine a difference between my ripped CDs that are stored in my server's SSD and the same tunes played through Tidal (non MQA).  I get more of a noticeable change from a cable swap.  Once I reached the current level of digital playback equipment that I have, the convenience of having no physical media outweighed any small sonic differences.  I do plan to try Qobuz soon and look forward to hearing if there are significant sonic differences between that and Tidal.

At least with my system, I've found that the original source material largely rules.