I am trying to set my things to install stuff from AUR and saw on other posts that I should run sudo pacman -S --needed base-devel
It ends up returning 404 errors.
Sounds like your mirrors are out of date. See Pacman-mirrors - Manjaro
That is wrong
Only sync new packages by updating your package database and fully sync your system
pacman -Syu --needed base-devel
You have the answers above.
Can’t emphasize enough, always make sure the mirrors you are using are up to date and that your local mirror/repository/database are in sync. Reading the Manjaro wiki or searching this forum, you’ll often see pacman-mirrors and pacman chained together. There’s a one-to-one relationship to a package on the mirrors and the package definition (meta data) in your local database. When the mirrors are updated, your local database meta data become old, obsolete, thus the 404.
A side note, I ran into this recently, but to display what belongs in a group: pacman -Sg base-devel