This is referring to the Downloaded ROM.
From install page, https://images.ecloud.global/dev/FP5/, the section
Downloads for FP5
you can download either Official / stable ROM or Community dev ROM. The ROM you download is a .zip file. Your PC is likely to have a tool to extract or unzip a .zip file or archive.
Although for a different Fairphone (3) maybe reading this will make some of the terms more clear. [HOW-TO] Flash /e/-OS on Fairphone 3 using Debian based GNU/Linux.
- (You dug into history and found an unfortunate link regarding Easy Installer, let’s not go there … but FP5 was introduced for Easy Installer but it was withdrawn. So you were correct initially when you decided Easy Installer is not an option for FP5 right now.)