I can't recall specifically. It''s been a couple years now. If I had to say, I think I would get to the end where it reboots and the USB is removed but it would boot up to the previous OS rather than Arch. If you have any resources to point to, please do. I want to give it another shot now that we're talking about it
The arch install guide is pretty good. The only tricky part is the bootloader install, if you don't get that right the system won't boot. Also you probably want to disable secure boot in the uefi bios on your system.
https://wiki.archlinux.org/title/Arch_boot_process#Boot_loader
One thing to keep in mind. If you do install the system successfully, when it boots you just get a command prompt. Arch doesn't make any choices for you. You have to install your choice of GUI and useland tools on your own.