Ah, yay is an AUR helper, though I personally see it as a pacman helper as well. Link here. Some of the flags and options that can be used for pacman can be used for yay, thus, some of the flags in the aliases I use are actually for pacman. Anyways, on to the breakdown.

alias yy='yay -Y --needed --norebuild --nocleanafter --nodiffmenu --noredownload --nocleanmenu --removemake --sudoloop'

This one is what I use to look up for packages. The result of runnning yy «search term» would be a list of packages matching the search term and prompting the user on which package(s) to install.

alias ya='yay -S --needed --norebuild --nocleanafter --nodiffmenu --noredownload --nocleanmenu --removemake --sudoloop'

This one is what I use for installing packages. Useful if I already know what package I would be installing.

alias yu='yay -R --recursive --nosave'

This one is what I use when uninstalling packages. I usually check the package name with something like yay -Qi «package-name-guess» beforehand.

I actually don't know much about both yay and pacman myself, since the aliases were just passed onto me by the same friend who helped me (re-)install my system (long story) and set-up the aliases. Having looked all these up, however, I might make a few changes (like changing the --nocleanafter and --nocleanmenu options to their clean ones`).