A ring group is the answer to "just ring whoever is around". Three phones ring, someone picks up. The only interesting part is what "whoever" means, and that is a decision you should be able to see rather than infer from a drop-down label.
What the call does
When you pick a strategy, the interface shows the resulting ring order as a list of names. A label like "longest idle" tells you the rule; the list tells you the outcome.
Every group has a required destination for an unanswered call. The field cannot be left empty, so there is no path where a caller rings out into nothing. It defaults to the site voicemail box.
Drawn, not filmed
Everyone at once, or one after another in the order you set. Either way the rest stop when somebody picks up.
Most people never open the second column. It is one click away rather than spread across the first.
On every plan
This is included whichever plan you are on, with no add-on and no module to enable. Plans differ by how calls are distributed and how long the history is kept.