Off line 3V3 power supply with 3V3 LDO without a transformer
This application board does not appear to be using an isolated supply. That means that any point in the circuit can carry mains live voltage and must therefore be in an isolated (plastic) box. This design is really only for engineers that have experience with mains-live circuits.
The circuit to derive the 3.3 V consists of a pre-regulator (MOSFET Q5 and zenerdiode D11) and a standard 3.3 V voltage regulator chip. MOSFET Q5 takes care of most of the voltage drop. This can be done if the current that is needed is small (a couple of mA).
The ground of this circuit is a &quot;local ground&quot; meaning it is not connected to mains ground or even to anything else. As I said: this board needs to be inside its own isolated box!
The ground is &quot;made&quot; at the - pole of C3 (just after the bridge rectifier). The mains voltage is rectified and the resulting (high) DC voltage appears across C3. The - pole of C3 is connected (through R7, R8, R9 = 0.33 ohm) to a node that is defined as being &quot;ground&quot;. The circuit makes its own ground! Note that ground is just a reference point and as such you're free to choose whatever point you like (some points make more sense to select as ground than others).
My advise to you is to ignore this circuit (at least for building your own stuff, for education it is OK) and use a much simpler and safer solution:

use a USB power adapter, like one that is used to charge a phone. Use a USB cable to get a safe 5 V from that.

then use a 3.3 V regulator module to lower that 5 V into a stable 3.3 V. You can buy such a 3.3 V regulator module at many places. On such a module there will be a voltage regulator chip similar to U2 on the dev. board you show.