Sorry that no one has answered this question. I'm answering this question so people may come across it like myself.
A64 is the short hand for arm32_64binder. It is basically 32bit for armv8 CPUs which are capable of 64bit. The software is 32bit with a 64bit binder for 64bit capable hardware. An example of a phone running an A64 firmware is the Galaxy A6 2018. It is running an Exynos 7870 that has support for 64bit but the software installed is 32bit.
"system" is not an arch it's just a name added to some GSIs for whatever reason.
Hope that answer your question, I tried to make it as simple as possible. If you are installing GSIs and you want to know what type of GSI to choose then install an app called "Treble Info" from the play store.