ConstA WIDGET (see FunctionArgumentsPanel for why both halves of the Insert Function dialog are shaped this way): it renders no buttons and does not close anything. The catalog is the SDK's, through CommandContext.FormulaFunction — so custom (workbook-defined) functions appear beside the built-ins, no category table is maintained here, and deprecated/hidden functions are filtered out by the SDK.
The summary + signature come from the SAME components the in-cell editor shows while typing
(FunctionSignature / FunctionPreviewContent), so a function reads identically
wherever it is met. Here the per-argument prose is suppressed (showParameters={false}): this page
answers "which function", not "what do I pass it".
Pick a function — search or browse by category, and read what the chosen one does.