To select part of a contour, see Selecting Segments.
To select the whole contour, double-click on the contour or Alt-click on 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.