The archive pulled for creating that package seems to contain the application already compiled.
If you are interested in compiling an application from code, you should rather look for -git AUR packages, those usually having open source code. Then, first try simply compiling the code – this will give you a compiled application – before looking for how to package that application.
You can usually look at the PKGBUILD in the AUR to see how that package does it. It’ll show every step in the process. I’d still recommend doing it with makepkg either way, since you’ll get a package that’s managed by pacman, rather than just files scattered wherever the app makers felt appropriate (usually /usr/local. Usually).
This app in particular doesn’t seem to provide the source, so you won’t likely be able to compile this one yourself.
That is what a PKGBUILD is - a description of what to do, how to do it and where to put stuff.
It contains all the relevant information (the steps taken) to install the software - whether it is by
unpacking a .zip file containing the pre-built software
(as is the case here)
unpacking a .deb file containing the pre-built software
unpacking a .rpm file containing the pre-built software
or building and installing it from source.