Indeed - I've seen more people recommend Hannah Montana Linux (apt-based) than any of those for newcomers recently.
You are entirely right that a Linux distribution is really just its package manager, the default packages installed, and some remote repositories which may (or may not) have had some customisation applied, which will have been pulled and built from a source repository somewhere. All that's really needed to swap between eg. Arch, Manjaro or Cachy is to update the repo files and issue a package manager update command, although I'd probably like to verify my backups and get a stiff drink first.
The House of Linux is built out of bricks, and the bricks aren't that scary - you can take them to bits and look at them if you like, they're usually zipped-up folders of text files and the binaries you'd get from compiling them yourself. But if that's not what you're used to, then yeah - 🤯 .
In all seriousness, I wish that most distros had art half as good as what Void Linux has - got some really gifted people, there.
Indeed - most Java IDEs have FernFlower built in, so it's dead easy.
Decompiled Java is surprisingly close to the original, especially compared to eg. decompiled C++; good luck with that. You get all the class, function and variable names back on the original line numbers.
What you do not get back is any comments. So you can see what and how, but not why. Admittedly, most comments are kind of useless and do not explain 'why' very well, but for weird-but-critical code they can be essential.