It’s actually not that difficult.
Complicated = made of lots of parts, but “logical and rational” — like a car engine, 10001 piece jigsaw of the Gobi desert, or (dare I say it) a heart
Complex = constructed with pieces with varying and incalculable influences, but often with a predictable ‘overall’ effect — like a flock of birds, a game of rugby league, or the initial diagnostic conversation with the parents of a child with leukaemia
The difference is important. In the former, we can try to understand and manage the pieces to affect the overall outcome. In the latter, we can fiddle with the bits but it’s effect will need to be seen rather than predicted.