All distro's differences come down to how the chain of utilities is stringed up together. You have:

• Bootloader
• Kernel
• Init and service daemons
• Package manager
• Display server
• Window manager
• Widget toolkit
• Desktop environment
• User applications

And a whole lot of in-between. Essentially Fedora and Debian each have defined and originated a set of core software that work as standards for the first 4 parts of this chain. Arch is another, even on pure Arch a wizard installer has to deal with those in order to set up a properly working system. For some, those are the most technical and difficult parts of setting up and designing an OS. Then every distro is a variation on the rest of the chain or customizations on the first few parts, but almost always based on one of the —current— three standards.

There are also philosophical differences that drive technical decisions in the background. Favoring one way of doing things over the other. Debian is usually focused on stability, reliability, security, function over form. Arch is usually about the bleeding edge, speed, max efficiency, innovation, customization, user freedom. Fedora is pragmatic and down to earth, compromising between the two and focused on smooth user experience. Usually different distros will provide some variation or adaptation on those themes. Like making Debian more corporate, or updated, or making Arch easier to install, or making Fedora but optimized for gaming, etc.