Interesting discussion - because I have the same question.
Lots of confusion in the discussion as well.
First, ALAC vs. FLAC:
Both are mathematical compressions, which means: They are reversible. When you decode the compressed file, you get the exact same bit sequence you had before. This means: Neither one is better than the other - because you’ll get the exact same file back from both, you won’t lose anything.
This is why both are called “lossless compression”.
MP3 or AAC are different: They remove ‘inaudible’ components of the music (e.g. a triangle being played while the guy next to the percussionist blasts his trumpet). Therefore, MP3 or AAC compression (and all the TV formats like Dolby Digital, Atmos etc.) are considered “lossy”.
The format of the music inside the ALAC or FLAC file
The “payload” of an ALAC or FLAC file does not matter - the compression algorithm of both does not even look at it. So, it will never change the bit rate or the sampling rate when you compress a file with FLAC or ALAC.
In theory, you could be compressing a picture or a Word document with FLAC: it’s purely mathematics.
What makes an ALAC or FLAC file a music file is the fact that all meta data (tags like title, artist, album, or cover art etc.) are kept outside of the music payload - which means that you can see these things, e.g. in Apple Music without decompressing the file.
And when you start decompressing, you will get your bit-perfect music payload back.
(One of the key differences between ALAC and FLAC is the storage of metadata: ALAC uses the same format as the lossy AAC/M4A files. FLAC has a different metadata format which Apple does not support, it does not mandatorily follow any standards.)
High-Resolution Music:
The generally accepted definition is: Everything better than a CD is considered Hi-Res. A bit rate higher than 16 bit, and/or a sampling rate greater than 44 KHz makes the file Hi-Res. 24-bit/44 KHz can be considered Hi-Res, and 16-bit/96 KHz is also considered Hi-Res. So technically, Apple's "24 bit, 48 KHz" limits can be considered Hi-Res.
Common Hi-Res files (like Apple’s) typically have both high bit rates and high sampling rates (e.g. 24bit/96 Khz or 24 bit/192 KHz).
That said…:
A FLAC or ALAC file is always lossless - but only as lossless as your original file.
If you re-encode an MP3 file to FLAC, you won’t get the information back that the MP3 encoder removed. You only will get the original music information from the MP3 file back.
If you encode a CD (which technically is a 16 bit/44 KHz WAV file), your FLAC or ALAC will get the exact original CD quality.
Apple:
Apple is offering “Hi-Res” lossless music in their store: It’s encoded in 24 bit/192 KHz. For delivery, it is losslessly compressed using the ALAC algorithm.
Apple apparently does not offer any device that makes use of these Hi-Res files: AirPlay2 maxes out at 24 bit/48 KHz. So does the DAC in the lightning-to-3.5mm cable. Not even the expensive Apple AirPod Max headphones can go beyond 24 bit/48 KHz. The AppleTV would have been the perfect device to allow passing on high-resolution music: In many cases, it is connected to an AV receiver over an HDMI cable. HDMI can handle the high bitrates and frequencies - and pretty much any AV receiver has a potent Digital-to-Analog converter inside. You could keep everything Hi-Res until you are converting back to analog.
Opportunity missed:
From what I read here, Apple latest ATV 4k still does not allow the output of Hi-Res files in Hi-Res over the HDMI output.
The passage about ATV digital audio maxing out at 24 bit/48 KHz was in Apple's documentation for the previous generation of ATV 4k - that’s why many were hoping that Apple would improve on this with the new device and its higher processing power.
The bottom line, I guess: We have to keep waiting. And I will not subscribe to Apple Music - because the biggest advantage it has - the Hi-Res files - evaporates the moment it reaches an Apple device.
