The frontender creates the visible part of the resource - the user interface. Backender - invisible, server.
Frontender must know HTML, CSS, JavaScript. Backender - one or more programming languages, MySQL and other database management systems, HTTP and HTTPS data transfer protocols. Both areas require analytical skills.
Junes, middles and seniors in both directions receive approximately the same. But the competition is higher in the frontend.
Frontenders monitor the release of new frameworks and libraries. Backenders need to know math. Another difference is the result of labor. The frontender always sees what he just coded. Bakender - no.
Developers are not taught at the university, you will receive only fundamental knowledge. Online courses will help you learn practical skills from scratch, create portfolio projects and find a job.