@finiego I’d seriously suggest getting a multimeter(maybe borrow one from a friend). It must be able to measure DC voltage (just about every digital multimeter does). Connect the old PSU to power (doesn’t have to be in the PC) and measure each pin. You need to find one GND and put the black probe on that. Then go over every pin and check with the red probe. That should read either 12V, 5V or 3.3V (only on the new PSU!). Write it down, I like to draw a picture of each plug and write voltages on each pin, with arrows.
The do the same on the new PSU. Once complete, compare the readings and see if everything corresponds.
The old PSU only had 12V and 5V, while the new one also has 3.3V