When a Tinderbox 7’s composite is selected, or even when multiple notes (not necessarily composites) are selected, the $Text of each of these notes is displayed en bloc in Tinderbox 7’s text pane. With Tinderbox 7 all of that text can be selected – Edit > Select All or ⌘A – and the text copied for use elsewhere.
The selected text is also available to macOS’ Share feature – that works with applications that support Share extensions. You can use Edit > Share or ^-click and choose Share from the contextual menu, and send the selected text to another app.
For some reason, the Edit > Share submenu offers fewer app/share extensions to choose from than does the contextual menu Share sub-menu. So, until Apple fixes that anomaly, the contextual menu is the better choice for sharing text from Tinderbox with other apps.