Download iso & signature from https://archlinux.org/download/
In my case archlinux-2021.08.01-x86_64.iso.sig and archlinux-2021.08.01-x86_64.iso
Verify sha1 checksum (replace archlinux-2021.08.01-x86_64.iso with your image path, image can be in download directory)
cd ~/downloads # optional
sha1sum archlinux-2021.08.01-x86_64.iso
# 4904c8a6df8bac8291b7b7582c26c4da9439f1cf