what exactly is seo career?

Just working in SEO as a job, either in-house for one company, at an agency with several clients, or freelance. Entry roles are usually SEO executive or assistant, then specialist and manager, and most people drift toward technical SEO, content or link building as they go. If you're thinking of getting into it, a small site of your own teaches you more than most courses. Being able to say "I took this from zero to X visits" carries a lot of weight in interviews.