Currently JekOS isn't as much an operating system as it is a way to bootstrap an install of NixOS to AWS in a browser. This bootstrapped "kernel" instance is planned to then takeover provisioning and monitoring of further servers/resources.
The installer is available at jekos.org but the AMI it references is not generally available (although the code has the necessary Nix expression for building your own). It probably won't be released until I can get jekos.net whitelisted by Let's Encrypt or I would quickly exhaust the 5-certificates-per-top-level-domain-per-7-days limit.