Unless someone knows a special command or access key to do this (I saw this same issue reported elsewhere to no avail, apologies if there is already something), despite using Dolphin for several years: 

When normally browsing folders using Dolphin's detailed expandable-folder view (with detailed information columns and Expandable option enabled), I cannot find a way to either:

1) Expand all selected folders down one level
3) Expand all selected folders recursively
2) Expand all folders down one level
4) Expand all folders recursively

In other words one can only expand a single folder by clicking the ">" icon left of it, and it does not do so recursively. Such a functionality is mimicked in IntelliJ IDEA's search results and other such programs, which have a button-based "Expand All" search results, and by the "tree" command-line utility. 

If there were a right-click menu item(s) (with sub-menu if need be, corresponding to the possibilities above), then one could trigger the function on all selected folders or using a predefined "Expand All [Folders] [Recursively]" menu item.

A programmable quick-access key could also be useful.

The general use case is to have a UI version of the "tree" command-line utility. Dolphin is the fastest and best file manager, therefore the best-suited to expanding large directory structures via UI. In general this is a more effective way to get a bird's-eye view of a large file set with minimal effort, thus increasing productivity.