Linux

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 it's not quite as "from scratch" as some people make it out to be. It definitely can be more work than most distros, but mostly it's pretty straightforward and you generally just follow some wiki guide or another when picking "components" to install and I think nowadays you can use prebuilt binaries too (I haven't used Arch in a good while.) There's really not all that many moving parts, most of them are sort of a package deal – if you want to use X, you have to install Y and Z. It really is more like building a PC from parts.

Now, if you actually want to build a Linux system from scratch, there's the very surprisingly named Linux From Scratch project

Arch is not really a DIY distro and certainly not for noobies. Those saying so are either naive or a troll. Arch is an excellent resource and it has its uses. Arch assumes you have a full understanding of a POSIX operating system and all components. Arch has an encyclopedic wealth of great information. What Arch is not, is beginner friendly, or remotely tutorial. If you try to use Arch to learn how a POSIX system works, you are going to have a very bad experience. This is about like me handing you the encyclopedia and telling you to go learn physics.

For a beginner, start with Fedora or Ubuntu because they won't wreck your system. Fedora is on the cutting edge while Ubuntu is stable — which only means that most packages are frozen and will not stay updated in the latest version. It means many things are old and outdated, but you can write a high level script that will never be broken by some change that happens in a software library in a random update. Windows is also a stable (outdated) operating system so that companies can write software that will not get broken because they fail to maintain it regularly.

Gentoo is a tutorial distro. It is the compile everything yourself and learn how everything works distro. Gentoo must be kept up to date, but if the Portage package manager needs input from the user, the Gentoo packagers layout the details in a very approachable way, assuming you are competent enough to make it through a Gentoo installation in the first place. Most people take days to weeks to make it through their first full Gentoo installation and configuration. This is guided, but there is an enormous amount to take in and figure out. You'll be compiling and configuring everything from the bootloader and kernel all the way up. If you are at this level of competence and understanding, where you can run Gentoo, THAT is when you should consider running Arch. Arch is basically Gentoo, but without the compiling and configurations. All the components are easily accessible in binary form.

If you run Gentoo for awhile and you really want to understand what the packagers are doing on an even deeper level, then do a Linux From Scratch build (LFS) this is a thing too.

If you want to learn in situ and actually use a system, use Fedora, get The Linux Bible if you want to learn sysadmin, and leverage the RHEL documentation for more advanced stuff because REHL is the original distro, and where a lot of the core kernel devs come from. RHEL is down stream of Fedora.

Every distro has a distinct reason it exists. It is foolish to turn them into team sports like brands. A key part of the learning curve is figuring out why each distro exists and leveraging that knowledge. I use them all for various reasons and know which to use for documentation. I may use Fedora as my base, but I have Arch, Ubuntu, and Gentoo that run in containers on my host machine, and have run all three as my base system in the past.

I was pondering on building my own spin of Arch. Have a look at this: archiso

You don't really build it from scratch. It's just very customizable. And it's usually installed by typing in the command line instructions yourself to do every step of the installation. That also gives you customizability. Other distributiins have a nice installer, you get some default options and click on "continue" and it does the same thing behind the scene. Like partition the harddrive, choose the default software, write it to disk, configure things and install a bootloader. With arch you do that yourself on the command line. They wrote some good documentation to help you. And you'll learn quite a bit in the process.