Widgets were an InDesign specific cross-OS framework where earlier versions would directly talk to MacOS Classic, Carbon (pre OSX Cocoa) or Windows.
Nowadays most of that UI is emulated, using another abstraction. You may have come across keywords and prefixes such as Drover, DV, OWL, or Drawbot – libraries shared by most Adobe applications, and the InDesign SDK does not provide headers for them.
There is plenty UI that does not use the boss objects. The application frame (as you were asking), ScriptUI, UXP, also at a smaller scale the tool tips, or those blue popover annoyances.
InDesign does provide selected few related methods, e.g. have a look at PaletteRefUtils or WidgetUtils (for Drawbot).
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