How do you control a control wire?
Yes you might use a switch to control a relay.  In a more complex circuit you might arrange for relays to control each other; in this case you can perform the elementary logic functions: inversion (where a normally-closed contact disengages a relay), an AND operation where several relays are in series and all must be energised to switch on the ‘output’ relay, and OR where several relays may be connected in parallel.  In principle the same applies to triodes and transistors, although the implementation is somewhat different.  Even the most complex digital circuits are made using these elementary functions.