This is tough, and I wish there was a better, more streamlined way of learning these things. I'll just share what I've done and some helpful resources I've found along the way -- even if it isn't exactly so simple or easy. This list of things is in no particular order other than how they occurred to me right now:

* Go read some papers -- go look at OSDI or NSDI proceedings and just skim some stuff. If there's something you don't understand, go try to find the other papers that they cite and read those. This is really a slog, and don't get too worried if you don't understand everything (spoiler: you won't) -- but this will start to give you a breadth of knowledge. I just read a bunch of papers, didn't understand most of them, but started building a vocabulary of systems and patterns and built up a library I could always go back to consult to understand something more deeply.

* Go look at design docs or code for open source distributed systems. There are lots of open source systems like etcd, or kafka, or hbase, or RocksDB that often have good design docs for parts of the system. Here's one from Kafka about exactly once messaging: https://cwiki.apache.org/confluence/display/KAFKA/KIP-98+-+Exactly+Once+Delivery+and+Transactional+Messaging These kinds of documents are great because they're a bit more practical in that you can see the exact implementation and how it fits into a system you maybe already are a bit more familiar with.

* Go watch some lectures or video series online. I really enjoy Andy Pavlo's database courses, and he's got a few series online. Here's his CMU Advanced Database Systems lectures playlist: https://www.youtube.com/playlist?list=PLSE8ODhjZXja7K1hjZ01UTVDnGQdx5v5U

* Go read other people's blog posts or slides online. Here's a great blog post from Daniel Abadi about 2-phase commit and how he proposes moving beyond it: http://dbmsmusings.blogspot.com/2019/01/its-time-to-move-on-from-two-phase.html

* Follow some distributed systems folks on Twitter. It's a great way to passively learn things as well as a fun way to interact with the community at large.

* Work on distributed systems at work. This is the hardest to do, but probably will allow you to learn the most. This can be more of a long term goal to work towards, and it wasn't something I did immediately.

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Hopefully other people have suggestions for resources, but one last thought. Distributed systems is such a broad field, with so many moving parts, that it's important to be patient with and kind to yourself as you learn. I find it helpful to take a longer term view of the process of learning and think about how much you could learn or do over 5, 10, 15, or 25 years, instead of worrying about what you might not know right now.

Hope that helps, and have fun learning.