The following are the crucial skills for senior Full Stack Developer:
HTML, CSS, JavaScript
Familiarity of one or more third-party library like ReactJS or Angular
Programming languages and libraries like Ruby, PHP, and Python
Experience with databases
Version control
Knowledge of security concerns and best practices
Knowledge of web or visual design and user experience best practices.
If you want to become a full-stack developer, check out this Full stack Developer Course by Intellipaat.