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!