What do these letters, numbers and symbols mean?
In English-speaking countries, the notes are named by letters from A to G. The number after the letters and symbols are the octaves. An octave is 12 semitones, and starts at C. The notes go in this order: C, C♯, D, E♭, E, F, F♯, G, G♯, A, B♭, B, and back to C on a new octave. As for the signs, ♯ is read as "sharp", and means "The note plus 1 semitone", while ♭ is read as "flat" and means "The note minus 1 semitone". Therefore, it is not wrong to write, say, D♯ instead of E♭, since they are the same note, but written differently.

Why do the significant low and high notes don't start at the same note for each member?
Basically, not everyone starts struggling or requiring more effort at the same notes. That's due to their placement. The placement is usually used to define where the voice resonates, but I've also seen it used to define where the voice naturally sits in terms of pitch, and that differs from person to person. Therefore, when I say "TOMOMI is higher-placed than MAMI", that means TOMO's voice is naturally higher than MAMI's. The placement can also be used to define the singer's voice type. For women, from lowest to highest placement, there is contralto, mezzo soprano, and soprano. For men, it's bass, baritone, and tenor. That being said, not because you are higher-placed than a mezzo soprano that you necessarily are a soprano. You can be a high-placed, mid-placed or even low placed mezzo soprano, instead. So voice types and placements are fairly nuanced.

Why are there questionable notes?
While every note a singer produces can potentially be part of their vocal range, it still has to be of sufficient quality to be "counted". There can be various reasons for which we don't count a note. The most common case is notes being too short, which can mean multiple things: lack of control, voice crack, accident, inability to sustain it, etc. The second common reason is that we can't identify for sure who's singing. A third common reason is the note's weakness, resulting in too little pitch for countability. There is also pitch-shifted notes that we don't count, because they're not produced by the voice directly, but by a software or machine that shifts the pitch of the note for the singer. And sometimes, the reason isn't as obvious… For instance, spoken notes in a non-musical context don't count, because their pitch is usually not really controlled nor purposeful. Comparatively, rapping or spoken notes in a musical context are more purposeful pitch wise, that's why these can count if strong enough. But inhaled notes don't count because they don't employ the vocal chords in the same way as exhaled notes.

Why are there notes in bold?
Bold notes are usually considered as impressive for the singer, but I like to consider them as great or representative examples of the singer's vocal abilities as well. It can be because the note has amazing tone, great control, long sustain, or is iconic… Which is admittedly, to a certain extent, subjective. By the way, the bold notes are all linked, so you can click them if you'd like to check them out!

What does it mean if you have a big vocal range? And what would a big vocal range, to begin with?
An average vocal range would be around 2 octaves and a half that are usable, because most of us won't master and push our voice's limits too much or at all. So 3 octaves and more is starting to be pretty big, and approaching or even surpassing 4 octaves is rather huge. However, if a singer has more than this average, it doesn't necessarily mean they're gonna be a better singer. It sure means that they have more possibilities (which also depends on the variety of vocal techniques used), but the quality of the singing is something else. See it as quantity not necessarily being synonymous with quality; it can be, but isn't always.

I hope this will help to understand what's written below!