When opening an asset using FSimpleAssetEditor here is a generic overview:
It will register the Details View, call FAssetEditorToolkit::InitAssetEditor, FAssetEditorToolkit::RegenerateMenusAndToolbars.
The tab manager is set in SStandaloneAssetEditorToolkitHost::Construct created inside FAssetEditorToolkit::InitAssetEditor.
The tabs registered to the tab manager are spawned from SStandaloneAssetEditorToolkitHost::SetupInitialContent called in FAssetEditorToolkit::InitAssetEditor.
For example the details tab is registered with RegisterTabSpawner in FSimpleAssetEditor::RegisterTabSpawners.