OK so when you install Linux on a system that contains a Windows system already, most installers will recognize that and configure your boot loader (e.g., Grub) so that it has a "Windows" item. That's because it's easy to identify the windows EFI partition and to reference it.

This way you tell your bios to boot on the Linux EFI partition, which will launch the bootloader, and you can launch Windows from there.

Arch being Arch, it falls on the user to ensure the proper configuration: after install, one should make sure that (example with grub) package os-prober is installed, the /etc/default/grub config file doesn't contain a line explicitly saying not to look for Windows, then run grub-mkconfig. This is true with archinstall too.

Now one can always decide that what should be done is to remove the windows drive (because they're afraid to mess it up when installing Arch), plug it back after Arch install is complete. Each system has its own disk and no system knows the other one exists.

But the BIOS still sees two bootable partitions and offers to launch either one of them. So these people who are so good at Linux literally boot Windows as they would boot from a thumb drive (smash F12 or whatever key at startup) as opposed to using the solutions that are provided, because they actually require to tinker a little bit. That's from the same people that claim the Arch Wiki is superb (it's got many flaws).