Basic information on alsa needed

I'm a bit confused about where & how 'alsa' fits in the "sound system" 
on my Linux computers.

I struggled for a long time to get sound working on my computers. I 
finally heard of 'pipewire', and finally I can now play music on my 
systems.

I'm now been using 'cmus' for a few months, and very much enjoying that! 
However, I've been comparing the configuration between two of my Linux 
systems (one has 'cmus' installed, the other uses 'mpg123'), and the 
differences are absolutely baffling to me.

Both systems are Debian-based, using the 'bookworm' release. One is a 
"Raspberry Pi Zero 2W", the other a "Raspberry Pi 3A+". I opted for the 
"Lite" version (aka headless) of the OS as these are lightweight 
computers. The "sound system" consists of a single Bluetooth speaker for 
each system.

Initial questions are these:

1. What role does 'alsa' play on my systems?

2. Is alsa's role determined at all by the installation of 'pipewire'?

3. Why would two identically-installed OS (on the Pi Zero & Pi 3A+; both 
with 'pipewire') have such different sound configurations?

4. Is there any intelligible documentation that discusses these questions?


Thanks!

~S