Many computers aren't  A mobile phone needs a speaker, microphone , keyboard and a little screen for user propmpts. Optionally it can have a camera chip and a better screen. A professional camera usually doesn't have the microphone, speaker, or keyboard but has better camera chip and a zoom lense assembly.

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The arithmetic logic, program control and all the other parts are built up from logic gates which do  simple logic operations.  In a recent binary electronic computer the gates in turn are usually made from transistors which are individual switches with an on/off action.

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