Just to confirm, for connecting the caravel breakout board to the caravel hat board, we need to solder the connections right ?

This version of the hat board is using flexypins, i.e. spring-loaded pins that are soldered to the hat board such that the breakout board can be inserted or removed without soldering. Here is a close-up of the connection mechanism with a different pair of boards:

Thanks. I managed to insert it without soldering. The breakout board pads have grooves where the flexipins fit in. Though it's a bit tricky initially.