In Bash you can use a shell option to alter this behavior: shopt -s nocaseglob. See https://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html for more options.

Maybe, but there is always the possibility that Downloads and downloads both exist in that path and in a case sensitive file system, those are going to be two completely different directories, so adding that obfuscation on top might wind up biting you later.

Sorta. If you put a FAT32 disk or sd card into a Linux system and mount it, it will ignore case because of the way the filenames are stored in that filesystem. However, there are a lot of important features you lose working on filesystems like that, so really it should be reserved for sneakernet with other operating systems.

You can use casefold option on ext4.

Symlink ?

ln -s downloads Downloads

or

ln -s Downloads downloads

depending on your situation.

I've kind of just accepted this is one of the differences between Linux and Windows that we as users need to understand is OS-specific.

ln -s Dowloads downloads

I guess you could use an ntfs filesystem... Or if you just mean for autocompletion, I've found that if there's no completions matching e.g. readme then zsh will autocomplete README. But I'd say case sensitivity of files is a feature not a bug. People use it to make files starting with a capital letter appear at the top of a list of files in a directory.

alias downloads="cd ~/Downloads"

edit: but if you want to get freaky in bash, alias downloads="pushd ~/Downloads"

probably works in some other shells too