Senior devs are the devs that are required when the problem is to difficult. They have fluency in many different technologies and can adapt their knowledge to other technologies. In the foreign language domain, they have a C1+ level in at least one foreign language.
They can be project managers and macromanage an entire team. They know difficulties and limitations (technical or not) of the client and they tend to do whatever they can to eliminate these "problems".
They understand the tech skills of the team and are able to delegate work for each person individually. Knowing when to give good feedback or code review.
They know how important is quality control and their work is usually accepted in the first review (a rate that is more than 85%). Always including unit tests in their code base that reduce a lot the quantity of bugs.
In the backend, they have a good infrasctructure fluency. They know and apply the concepts of deployment automatization, capacity planning, security, tuning and perfomance.
They are open to teach other devs and give better ways to implement methods, or other solutions. With customers, they are excellent to find good solutions that have a high cost and benefit ratio.
They are always up to date with the software development world, knowing how to identify good devs or professionals.
They know that scaling is to improve the team productivity.