A senior developer in my book is someone with both leadership skills and the experience to back that up. Most of your work is enabling other developers with a bit of actual dev work sprinkled in.

You're still chiefly a problem solver like any dev, but your problems now are more than code. Hence, your toolset expands to implementing new processes, talking with non-devs, creating PoCs, teaching, and more.

Here's a roadmap I made to help out: https://github.com/glennsantos/senior-developer-roadmap/