And when I tried termux on Android, oh dear lord. The whole thing breaks because of permissions issues. Had that issue on WSL too.
Lowest-common-denominator for linux and mac, mostly just "make it work if you notice it's broken". I have one file that's bash magic, and link it to everywhere it might be needed - bashrc, bash profile, etc. It's kind of monolithic, not great but at least 100% consistent for what ends up where, and avoids sourcing external stuff or requiring N different files to work. You'll see some of your pain in the repo history, change X to Y to make it osx or *nix compatible based on whichever is my primary platform.
Same goes for vim, just absolute basics for vimrc and a few very portable plugins and niceties hardwired. No package manager, just a bare directory.
Lot of other suggestions on the same HN thread: https://news.ycombinator.com/item?id=11070797
(Disclaimer: I am the author of vimswitch)
This is what I do, too. Most of mine is portable, but I have a block in bashrc that branches on uname.