From front-end to full stack. What and where to learn?

I'm self-taught front-end (React) developer and I want to go full stack. What (and where) should I learn to get a job?

A bit about me: I understand programming quite well (e.g. I understand functional programming and lambda calculus). There's no problem for me to learn a new technology (language, framework). I'm asking about abstract concepts.

What are the most important things for a back-end developer? Databases? Some design patterns or maybe algorithms? Something else? Are there any good books I should read?

(It would be great if examples will be in JavaScript or Python but that's not the point)

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You may skip this section, it's about my computer-science interests. That's not the point

I really like the idea of functional programming. I'm interested in programming languages theory, compilers, types and formal systems like lambda-calculus. And I also dream to work on something important for humanity (like bioinformatic)

Anyway I don't really think I can get a job there, at least now. I think realistic approach is to keep moving on web development and go full stack (not just back-end because I actually like front-end)

I write this section because maybe some of you have an idea, what should I do with all this madness :)

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Thank you all! I'm glad to be a part of you