Nope, it's a bone. Ligaments are flexible.

Basically, your pelvis is made up of a bunch of smaller bones that grew together. So there's bits that poke out, and bits with weird shapes, and doctors give them names so they can talk about something besides "that pointy bit". Most of those bits were separate bones when you were a baby, and are all part of the pelvis now. The default advice is you "should" be sitting on your sit bones. But we've all got different bodies. If your butt is happy on some other bit of the pelvis, it is probably not wise to adjust things just to sit on your sit bones.

Wrist pain can come from a lot of different sources, so there's no one size fits all answer. For the new and interesting pain that kicks in for me on some 20 mile rides, the right solution is going to involve bar ends or different handlebars. The flat handlebar on my bike doesn't let me have a neutral wrist. My hands *really* don't like that, and will screech at me if I am foolish enough to knit with bent wrists, spin with bent wrists, type with bent wrists... So it's no shock that biking with bent wrists is bad too. I had some hand and wrist pain while I was getting my saddle situated early on... and that got fixed by a better saddle for me, and pushing my saddle further *back*. Bodies are *weird*.