yay -Syu : upgrades packages that are in the official repos
yay -Sua : upgrades only AUR packages
yay : equivalent to running both of the above commands (i.e., it’s the same as yay -Syu && yay -Sua).
I think you will find that most EOS users simply run yay.
yay -Ss is the search function, for example, yay -Ss vlc will search both the repos and the AUR for packages containing the term “vlc.”
Personally I use the web interfaces to search packages.
Arch repos:
AUR:
I assume you mean the cleanbuild prompt - usually a cleanbuild is not needed, that’s why the default option (if you simply hit Enter) is no.