In your professional evolution you start with simple tasks, learning to accomplish them, first under supervision, then alone. Then you get larger assignments and learn to work without hand-holding. Finally you learn to solve all kinds of problems in your field of expertise with your chosen technology and you get entrusted with making the right decisions and doing the good job of it.
But then at some point it stops mattering what work to do, in what part of the technology stack it is located, or even what technology stack this is. It becomes a matter of simply learning what you need to know to perform the work, doing it, closing this issue and moving on. That's what seniority means (or should mean to provide a base of reference).
This is viewed from the technical side of things of course, and I believe this is what the question implied. Good communication abilities and developed soft skills are essential too but those transform a developer into somebody else to whom the title of a senior developer no longer applies. They're no longer just a developer but a higher-level product.