I've been mainly working with really simple CRUD apps. 1 server apps kind of things.
That's why I'm also afraid that I won'be fit and good enough to work on those big complex apps.
Can you look for a junior position in a medium to big company, or a startup, with enough engineers to help mentor you?
If you want to try it yourself, you can start by experimenting with database clones, redis shards, K8s, etc., and then try reimplementing the same things in various cloud providers, so you can get a basic understanding of a shared/sharded data model in various frameworks.
Or, if you want to leapfrog all that, you can jump straight to serverless stores like Firebase or https://www.cloudflare.com/products/workers-kv/ that abstract away the infrastructure and let you focus on writing business logic on top of that.
But IMO you will learn more quickly from one year on the job, during professional backend work in a capable team, than self-learning willy-nilly (where you can pick up a lot of bad practices, or outdated/inappropriate architectures).
In general I think working at (very) small startups is the best way for very capable, self-learner types to be exposed and allowed to do lots of things in a short time as it's basically a free-for-all matching tasks with anyone who can get it done.
Edit: after writing this, I realize that the other Ruby/Rails place I'm working at (Shopify) does let non-senior devs do some complicated stuff in the core monolith and jobs that gets reviewed by senior devs. The team I'm on runs Kanban-style 2-week minicycles (didn't like Sprint'ing) and encourage different people to try tackling different things, often with pairing or review of proposed approach.
More to the point: The tricky part of doing it all yourself is you have no idea if you're doing it well, and whether it would survive heavy real-world use and other devs/teams/companies/APIs/user agents. It's not easy for a solo dev to catch pitfalls that more experienced devs, or just more # of devs, might notice in a pair programming sesh or code review.
By all means experiment on your own, but IMO I found teams to be much more helpful for actually learning best practices rather than barebones spaghetti code. Startups count too! What you may lose in learning architectural best practices (depending on if you have any seniors and how good they are), you gain in problem-solving and research abilities, not to mention learning to do a lot of things at low cost.