I’m afraid Hydra is, like many other parts of NixOS, underdocumented. You can take a look at the wiki which links to the manual as well but it’s mostly about running Hydra and not at all that useful from a simple user perspective.
To complicate matters even further, other shortcomings make the search at the top right basically useless - it’s quite likely to timeout (see: search feature often times out and results in error 500 · Issue #901 · NixOS/hydra · GitHub).
The only advice I can offer is to describe how I located that iso: navigate to Hydra → nixos → release-21.11 → Jobs tab → search for iso\_.
For slightly different use cases - e.g. if you’re just looking for a package or configuration option I recommend using search.nixos.org which works really well for that purpose.