The total-sum Sound Quality of Phone music aps + Bluetooth

Hi there folks,
I am a 'funny old fart' that doesn't own or use a Smart Phone, but I am interested to know about the sound quality people are getting
using Phone music aps + the use of Bluetooth ???
EG. Are there differences between Spotify, iTunes and YouTube?
Hopefully some people can enlighten me 🙂
 
Here are my very rough estimates of how upgrades improve sound quality.

Going from YouTube -> ITunes: 2% improvement

Going from ITunes -> CD player: 5% improvement

Going from $200 amp to $10k amp: 20% improvement

Going from poorly designed speakers to great speakers: 1000% improvement.

Honestly Youtube over Bluetooth is absolutely fine unless you’ve optimized every other aspect of your system and need to eke out the last drops of performance. You’ll almost always see a bigger improvement by upgrading your speakers in my opinion.
 
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I found this simple 'Google link' regarding Bluetooth quality. I haven't yet read it all , but it might be interesting to some readers.
One thing I'm still not clear about regards > Audio headroom & Digital Zero dBfs. Is some form of 'Auto-level sensing' incorporated?
Although 'standardization' of volume-level is now closer than ever, we still find quite large differences in YouTube clips for example.
https://www.google.com/search?q=aud...BGAqYAwCSBwgwLjcuMTEuMaAH_4sB&sclient=gws-wiz