Sailfish, like Android, is based on Linux, but it’s important to understand that:
All that said, starting from Android is probably the best bet for any alternative like Sailfish, which is probably why the Sailfish Hardware Adaptation Development Kit requires you to start with an AOSP/CyanogenMod/LineageOS installation.
You can watch this video on Odysee and PeerTube, too.
Please note that you can't download a 4.0.1.48 image yet, you need to a previous version as described in the description of the video: Set a root password as shown in the end of the video. After this, open the terminal app and enter the following commands:
devel-su
ssu re 4.0.1.48
ssu ur
zypper ref
zypper dup