A few random examples (not the best you could find, just something I've used recently):
- re-packaging pre-built binaries:
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=visua...
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=nomad...
- building C from source
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=tinc-...
- building Go from source
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=yay
- patching and building a kernel
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=linux...