For backend engineering specific, some free & paid resources are
- O'Reilly Membership - This is a gold mine. For the $400 I believe you can purchase a yearly membership, where you get access to the entire O'Reilly catalogue. Designing Data Intensive Applications is included of course. They also have some video courses & conference talks in addition to the books. If you don't want to spend the $400 then they also offer a 7 day trial and don't ask for a credit card....
- quastor.org is a good read (but it's free). They follow all the big tech engineering blogs and send summaries of the interesting backend-dev blog posts.
- bytebytego - this is also free. It's mostly diagrams and provides a very high level overview but it's a good subscription. You can also purchase their books on their website.
- LeetCode membership - good for interview prep if you're looking for a FAANG-job, pretty much useless for everything else (could be helpful if you like competitive coding though!).
- Udemy Courses by Hussein Nasser - I really liked his course on databases. Delves into the different database engines, tradeoffs, query optimization, etc. He also has a YouTube channel with lots of free content.
- codecrafters - I haven't done this myself but it's a bunch of interesting challenges where you build a toy version of Redis, build a bittorrent client, build a toy version of Git, etc. Could be useful to understand how tech works. In terms of a free version, there's also (https://github.com/codecrafters-io/build-your-own-x) which is a collection of blog posts where you're building different things in various languages.