PKGBUILD is a shell script containing the build information required by Arch Linux packages.
Packages in Arch Linux are built using the makepkg utility. When makepkg is run, it searches for a PKGBUILD file in the current directory and follows the instructions therein to either compile or otherwise acquire the files to build a package archive (pkgname.pkg.tar.zst). The resulting package contains binary files and installation instructions, readily installable with pacman.
PKGBUILDs install regular files Linux Directory Structure