Arch Linux can be installed in many different ways. There is no one way to install and use it. The idea is that the user will customize it to meet their needs.
EOS installs the system for you but there is very little technical difference between an EOS system and an Arch system.
EOS has some extra tools to make it easier and/or more user accessible to do some things. EOS uses dracut by default. EOS has some automation for systemd-boot so you don’t need to manage the configs by hand.
Virtually all the packages in your EOS system come from Arch directly.
There really isn’t that much difference. If you switch from EOS to Arch, your system will not be different in any meaningful way. I run both EOS and Arch interchangeably.