The terms 'brick' and 'shoebox' are used just to relate to a known object of a similar size to the referenced 'unitary' PLC. Beyond that there is no similarity implied.
(OK - so they would both have mass and occupy volumn and they might be a similar color (depending on the brick or shoebox you may be thinking of) - but nothing else) ---
(OK - a brick might have some holes in it and the PLC might also but beyond that there is nothing implied) ---
(OK - a brick, if you just had it in an oven might be warm - or even a shoebox for that matter as long as it wasn't too hot, in which case it would just be a pile of ashes - and so might a PLC if has power and is turned on - but beyond that, nothing is implied.)
(OK - a brick, if it had Christmas lights wrapped around it - or even a shoebox for that matter, would have little (depending on the size of the bulbs I guess) glowing spots of light (assuming the Christmas lights are turned on) and so would the PLC if it was on - but nothing else is implied)