I think you misunderstand what Arch is. You absolutely do not assemble the entirety of the OS from scratch. You don't compile anything during an Arch install—you install pre-compiled binaries. And you don't actually have an awful lot of OS freedom in terms of what gets installed. If you wanted to use, say, openrc+musl+busybox+dracut, Arch wouldn't be for you, as Arch uses systemd+glibc+gnu+mkinitcpio (You can try to replace these but these are what Arch uses by default; if you're wanting to change these things, maybe just use a different distro). Arch just doesn't install a display manager (you don't need one; I don't use one), any kind of graphical session (you technically don't need one either, but I assume the vast majority of desktop users want a graphical session), or a bootloader. You can install all of those things yourself. Assuming you want all three of those things, that's probably just three packages you install, and the OS doesn't install for you, so that you can pick them yourself.
Arch doesn't have an installer insofar as you install it with shell commands, but also the actual install itself is just one pacstrap command which installs a full OS for you in one command.
If you're wanting to build an OS entirely from scratch, you may want to look into Linux from Scratch [disclaimer: I have not done LFS]. I don't know of anyone who actually daily drives LFS though, as you wouldn't have a package manager which would put most users off.
oh noooooo not the copyright infringement!