If it’s a PPA like Schism Tracker, then just unzip it to wherever you want in your “/home/(user)” directory. It only has an executable file and a couple of text files, which are the documentation. I’m only mentioning that program as an example.
It depends on the contents of the ZIP file and the packaged application. It might contain only an executable file, but not its dependencies. It might contain one library or more, but it wouldn’t mean that it satisfied all dependencies.
Some packages like Freebasic come with scripts to install it into “/usr/bin” and places like that. Use your own judgement about it. Sometimes there’s no need to “install” such an application, but you should unpack it in a folder somewhere inside your regular user’s account.
pacman or any other package manager cannot do anything about it, it’s up to you when you want to update such software. The same with AppImages.