I had a slow day at work in the beginning of my career (I was a help desk monkey then) and I wanted to learn more about how Active Directory actually worked. One of the things that I found out and became more curious about was why DNS was so critical for Active Directory, upon where I discovered the magic of SRV records. After that I decided I wanted to learn DNS in detail, so I read the RFCs, poked around with nslookup and dig, and then set up a local copy of BIND on a Linux VM to experiment with. In the numerous intervening years I've picked up more over time and been involved in numerous situations where I had to assist someone by doing DNS spelunking, often learning something in the process. I jumped from being a help desk monkey to being an L2 sysadmin in my very next job largely on the basis of my DNS and Active Directory knowledge, all of which was learned by reading documentation and poking at things, nothing particularly difficult about either.
What always causes challenges is weird edge cases, but if you understand how things are supposed to work at a protocol level you have the basis of knowledge to troubleshoot the edge cases, which is how you learn about the more esoteric or implementation specific behaviors of things. I wouldn't expect anyone to particularly have deep knowledge of AD or DNS edge cases, but the basics of how they work aren't particularly hard knowledge to acquire and it's a constant surprise to me how few tech people understand DNS in particular, since it's a critical system for nearly every aspect of modern computing.
As far as resources, when learning these things way back when I only used the official Microsoft documentation, the RFCs, man pages, and sites like nixCraft to learn about it. These same resources still exist (although Google sucks now and makes anything decent hard to find), and these protocols have changed very little in the intervening nearly 20 years, so I don't think it's any harder for folks to learn today than it was for me back then. This stuff is very literally not rocket science, DNS is an extremely basic protocol. In fact, nearly all its weird edge cases is because it's such a basic protocol that some behavior isn't clearly defined, however /most/ behavior is. It's very nearly all text on the wire, and all of the zone records, queries, and responses are human readable. As someone else mentioned, you can just use Wireshark and learn about DNS. It's inspectable on the wire, not just through tooling.
FWIW, Active Directory is far more difficult to learn in detail than DNS, largely because Kerberos is deeply complex, and Active Directory has many unique complexities (like how FSMO roles work). DNS in comparison to most other things you are expected to know as even a semi-competent systems administrators / SRE is mindbogglingly simple.