Modernists, at the beginning of the 20thC sense the chaos of the modern world (massive shifts in class,gender roles etc) and respond by trying to stick the world back together or resolve things back into a whole.
So Eliot in 'The Waste Land' puts working-class women waiting for their husbands to come back from WW1 next to classical myths and busy London streets and tries to make a story that makes sense of the connections between them. He says he is 'shoring fragments against my ruin...'
The pritt-stick response, glueing old and new, popular and classical back together.
Postmodernists sense the chaos too, but embrace it. They reject 'sense-making' and disrupt any big explanations or 'grand narratives' that offer great over-arching truths (religions, nations, patriarchy etc). They don't let any one story act like the 'truth.'
Sendak who wrote Where the Wild Things are is the son of holocaust survivors and is absolutely a postmodern thinker: brought up in a home traumatised by the Nazi attempt to make their 'Final Solution' come true.
An early 20th century 'design-led building might look like it is glueing styles together
But the postmodern Bilbao gallery looks like this
As you were ...
(Do please ignore me. I have a compulsive teaching gene)