This explains it pretty well - https://www.jomarpress.com/nagel/articles/Solfeg.html
I have to say that I find this pretty confusing because I'm european and I've internalized the Do, Re, Mi, Fa, Sol, La, Si scale ... so I can't related with all the fuss
What I can say is that I find the article kind of misleading as it makes examples as to why we need to not relatively but
absolutely recognize a note FORGETTING that you never start singing by knowing absolutely which note is what but by getting oriented relatively starting from the note La (the diapason)
I've yet to find someone who is able to sight-sing absolutely a Do or Mi whithout first listening to the La and tuning the note they need starting from it
You americans are making it confusing while it is actually rather simple
Ear-training is known also as intervals-recognition which makes it clear that it's not a matter of reading and singing the note but "reading and singing the relationship between one note and the next" (which is also the core of sight-reading at the piano: you don't read the notes you read the intervals!)
I have read the article but I still can't understand how she managed to make it that complicated.
Going by steps I would say that:
1) Music is not made by notes. Music is made by intervals. Intervals are made by notes
2) Therefore each musical concept related to "reading" depends on "intervals"
3) Sight-singing means that you take a sheet and sing what you see (i.e. intervals)
3a) it's impossible to do this without having a "note of reference" which universally is the La of the diapason
3b) I think that singing with the sound "ta" would work anyway but teachers prefer students to sing the "note names"
4) As sight-singing depends on "intervals reading" you practice sight-singing by practicing "intervals"
5) Intervals are relative. Their quality remains the same whether the tonic is a C or a F#
In other words: intervals CAN'T BE FIXED
6) That's why I find the Fixed Do vs. Moveable Do so confusing. It's a matter of "fixing" the notes absolutely or "considering" the notes relative but sight-singing and ear-training ... are not a matter of notes but a matter of intervals
6a) If one focuses on the intervals the Fixed vs. Moveable doesn't make any sense as the concept of learning intervals, reading intervals and singing intervals takes care itself of its methodology
7) So when I see a G - C I don't think about singing a G and the singing a C.
I think about singing a perfect 4th starting from the note G
The way I know the note G is by relatively finding it from the note La (which the diapason gives to me)
7a) Even thought I'm singing a perfect 4th which is relative and can start from whatever tonic and I could say Ta-Ta I'm singing a perfect 4th which is G - C so I sing this perfect 4th singing Soooll - Dooo
It's so simple. Why making it that complicated?
Here are two examples of solfege
The first is a "spoken solfege" and the second is a "sung solfege (sight-singing)"
Spoken Solfeggio MP3
Sung Solfeggio MP3