…normally does not have a PKGBUILD including the repo you’re cloning. Read the instructions in each repo. Some projects may be in the AUR (Arch User Repository), search there as well.
Would be nice if it was the “default gesture” to include at minimum a basic PKGBUILD template within the project. After all, the original developers know their software the best, and would lead AUR builders with something they can more easily work with, rather than from scratch.
The day that becomes the norm is when a new distro will emerge because “Arch Linux and its derived distros are too mainstream!”.
Too add to what @Yochanan noted, you can also read the included documentation (per project), which will hint to you what to do in your own custom PKGBUILD under the prepare(), build(), and package() operations.
Nice, but asking/expecting any developer to have to take additional time to learn/comply with Arch packaging guidelines (esp. if their dev environment is not Arch) is absolutely unrealistic.
There are also plenty of PKGBUILDs that you can look at for inspiration as well. There is also a sub-forum on the Arch BBS for help with PKGBUILDs as well.