To select part of a contour, see Selecting Segments.
To select the whole contour, double-click the contour or Alt-click white space near the contour. Use Shift while clicking further, to add to the selection or remove from it.
To select contours belonging to different elements, turn on Edit > Edit Across Elements.
To select contours belonging to different glyphs, turn on Edit > Edit Across Glyphs in addition to Edit > Edit Across Elements.
Use the Edit > Invert Selection (CmdI) operation to deselect the selected contours (nodes) and select the rest. This command works only within the current element in the current glyph.