I know this is probably a weird question but I am just wondering if it is actually possible for other people. I'm currently 15(and a girl) and my comfort zone is around the baritone voice type. Even my speaking voice is deeper than most people, and I've been trying to understand how this is possible in any way.
You can and (I hope) will sing wherever your range is comfortable. If it's where usual baritone or tenors sing, then so be it. What everyone needs to do is find the key for each particular song you want to sing. The key that suits your voice, for each song. It's not going to be the same key for every song, just because it happens to be D for several!
Most people have a two to two and a half octave range, roughly speaking. Some can extend to more, although the quality at each end may suffer. Karen Carpenter, I believe, had a four octave range - so could sing down where you are, but easily into soprano and more. Useful when multi-tracking!
Work with what you have been given, there are at least a million songs out there that will suit your voice, and if you sing in a choir, the lower register of your voice will always be welcome.
It's not all that unusual. Timbre is also part of the equation so even though there is some overlap between male and female voice ranges, "crossovers" in choirs tend to be rare unless nothing else could possibly work. In all-male and all-female choirs, the appreciated ranges at the "wrong end" tend to be quite more. Practice helps accessing registers and ranges that feel less natural and thus can make choir and ensemble work more accessible. For chanson-like solo singing, a low female voice responding well in chest registers will be very much sought after.
So not much to worry about, assuming that your usefully accessible range will, after training, be large enough to be musically useful.
Certainly. 'Female baritone' is almost the standard voice type for mature Broadway actresses!
Make sure you ARE singing, not just crooning along with a recording though. Many self-taught singers never actually 'turn the engine on'.
There is an actual name for us: We are typically classed as contra_alto (not the same as alto). Generally that means female voices of tenor range but baritone included in this as well. We are rare, the lower the rarer and more wanted!
The alto typically ranges from F3 to F5, where tenor ranges from C3 to C5. A baritone generally stretches from A2 to A4.
Practice! Strengthen. Don't forget, you are young and your voice may still drop (girl voices drop as well, just not as drastically).
I can sing nearly a whole octave lower than in high school but my higher range is less than it used to be since I stopped practicing. If you want something flirty, there's always Nina Simone.