Same way as with previous releases -press the buttons at bottom right that toggle them on/off. I think the main change is that they now default to on whereas under earlier releases they defaulted to off. When you hit the relevant button to hide a panel a yellow edge will appear on the centre panel to remind you that there is a hidden panel at that edge.
The other change is the wide/narrow setting under Preferences that determines whether the book details panel is at the right/bottom, but if you change that setting you have to exit/restart calibre before it takes effect.
One change I do not necessarily like is that when in 'narrow' mode with the book details at the bottom of the screen the toggle buttons are now underneath the book details rather than alongside it which wastes vertical space which is not so good if you are working on a lower resolution screen. I wound if a request for these buttons to be arranged vertically alongside the whit box used to display Library icons might be looked on favourably?