Package Cache

Clean the cache of old packages in /var/cachepacman/pkg/:

ls /var/cache/pacman/pkg/ | wc -l
sudo pacman -Sc
ls /var/cache/pacman/pkg/ | wc -l

And the same for yay (with -Yc to remove old dependencies):

ls ~/.cache/yay/ | wc -l
yay -Sc
yay -Yc
ls ~/.cache/yay/ | wc -l

New Configs

If you chance a configuration file, such as /etc/environment, and pacman wants to update the file, it will place /etc/environment.pacnew.

Check the new files, then look at the difference between the pacman version, and your version.

sudo find /etc/ /var/ /usr/ -name "*.pacnew" 
diff /etc/pacman.d/mirrorlist*

Either,

sudo -e /etc/pacman.d/mirrorlist
sudo rm /etc/pacman.d/mirrorlist.pacnew

Or,

use a tool like pacdiff to view the changes next to each other, and select them with vim.

sudo pacman -S pacman-contrib
sudo pacdiff