Senior means two things: - Have experience - Self learning though above experience. I met many experienced developers, but most of them failed at step 2. To me, they're still junior.
I would reformulate #2 as learning to figure out things on your own, i.e. not to bail out as soon as something is outside of what you’re familiar with. The ability to go one level deeper to find out how things work under the hood. The ability and interest to extend your horizon, to pick out the learning material you need for familiarizing yourself with something new, without needing it to be served on a plate by some…
More than once I've proven my value as a senior engineer by refusing to do work that should be left to someone else. Sure I could figure it, but it would be a waste of time when someone else can. Sometimes because it is obviously an easy task, and thus good for a junior engineer to learn to figure things out on. Sometimes because it is something I'll never need again and I know we can hire a contractor who already knows how to do it.