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
I’ve only done it the “manual” way but I don’t think the script was even a thing the last time I installed Arch, which must have been close to 15 years ago now.