Getting started as a back-end dev, what did you study that you now never use? What do you wish you had focused on?

Question

I watched a front end web developer's first day YouTube, and he said that he focused so much on creating CSS from psd in his training, but at his first job he learned react and hardly does css at all. I have more python, php, sql, access, and excel data experience and desire to work with the logic and scripting python magic. what are back-end things that are commonly taught to aspiring jr devs, then when you started working in web dev you went into a whole nother direction? what do you suggest knowing just enough or know a lot of for getting into a first job.