I'm new to STM32. Recently I'm trying to understand the schematic of it. I came across the term "soldering bridge" default: open. Does it signifies that the two pins are opened (which means not connected) or it means two of the pins are shorted?
It's two adjacent pads intended to be bridged with solder to connect them depending on the board configuration. It's just like a jumper with header pins and a shunt but is cheaper and more permanent.
Open is not connected. Closed switch is connected. In electronics, we use the analogy of a switch, not a door or floodgate (which is opposite).
It means they are normally not connected. You can place a blob of solder across it to connect if needed.
I would suggest it means "Not connected". Thus pin 4 on the P-channel MOSFET Gate has to be taken low to pass along U5V from Source to Drain.