At the front my raised beds are only about 4 inches deep and sit on top of clay. They fair much better - the roots get down into the clay, and the layer of compost on top keeps it moist and feeds the soil life. The only issue I had at the front is that the drought occured right at the start of the season when the roots of my annuals wasn't getting down below the compost. Most of the perenial shrubs at the front were planted directly into clay - years ago (before I was interested in gardening) - and they're fine without any water. Likely because anything that needs TLC has died off over the years - a case of natural selection!!
At the back the raised beds are built on a slope and they are about 18 inches deep. This is more problematic as the roots of many plants don't make it down to the clay. The compost drains quickly and the water runs down the hill. This is where I really need to think about adding in clay, sand etc. But I'm also thinking of doing something with ditches above and below the bed to hold water (based upon swales)
Trees, shrubs etc just go straight into 'the lawn'. The lawn being what used to be an old field. They do fine once established and rairly need watering even in drought. Although this year I have felt the need to water everything - last year trees lost leaves early and then we had a hard frost over winter that killed lots of long established trees and shrubs.
Nowadays I still slit plant bare root shrubs and trees directly into the clay. Alghough nowadays I mulch the surrounding area with compost or manure when I'm done. I also plant comfry 'south' of the tree to shade it from the sun. I then chop and drop that regularly onto the tree roots.
All of this is fine in what our climate used to be. But if the pattern of the last couple of years continues I will need to rethink. Trees all around here are losing their leaves in June it's so dry. Right now I'm watering everything, but it's a huge amount of work and likely we won't be allowed to do it if drought conditions become the norm. So I want my garden doing a better job of taking care of itself.
But whilst we're very hot and dry early summer, by late summer and through winter we have a huge amount of rain - constant so not just flash floods.