Adding Current limit for DIY DC-DC converter (CC/CV battery charging)
Current limiting is based on the fact that I = U/R
And the easiest way to reduce I (if it exceeds some limit) is to decrease U (voltage) which is what Great Scott is doing in his hacked video and explains it very well. 

It is also important to understand how op-amps works, since they are doing all the logic here. 

Typical simple DC-DC converter IC is an analog logic device. It knows nothing about what voltage on the exit now, what voltage you want to get  or what is the Current and Current limit. All it knows – is FB pin above or below Int. Reference voltage (e.g. 1.25 V) at current tick. If it above reference, e.g. 1.33 V with voltage divider 1:10, which means 13.3 V at output, it reduces duty cycle a little. Next tick it's 1.31 V, reducing duty cycle again, until FB is below 1.25 V (=12.5 V) in which case it will increase duty cycle and than decrease it...dancing around 1.25 V reference.

As did in the video, we just put 3.3V at FB pin (with additional ext. op-amp) when voltage is above some  limit, so converter will start to reduce the duty cycle and output voltage each tick, until the current will drop below set limit and our op-amp will dance around this current limit now. This called CC (Constant Current) mode, instead of normal CV (Constant Voltage) mode.

Btw, to indicate CC mode you can simply use current comparator's (op-amp) 3.3V output with a capacitor, resistor and LED.