Variable DC power supply

The bridge rectifier gives a continuous pulsating DC signal. Then capacitors are used to filter the pulsating signal into a smooth non-pulsating DC. Finally, the voltage is regulated using a regulator IC.
A variable dc power supply is one of the most useful tools on the electronics hobbyist's workbench. This circuit is not an absolute novelty, but it is simple, reliable, "rugged" and short-proof, featuring variable voltage up to 24V and variable current limiting up to 2A.

Can you provide any schematic or the internal circuit pictures? (Schematic is what that is mainly required).

But it is best to use proper buck-boost regulator additionally across the output for configuring it to a 0-25V power supply unit.

However, it can be done as far as transistor based switching device is concerned. It will be helpful if you provied proper schematic, if possible.

According to me directly using a variable resistor or potentiometer is not a good practice, because same thing you can do with a fan regulator. For this you need to attach a power supply regulator LM317, which is commonly used in DC variable power supply. Other thing you can do is that you can purchase a DC to DC buck converter for getting variable voltage supply. I suggest you to get more assistance from Sierra Software Ltd. technical support team about products and techniques.

Well, LM317 is also the way to go I thought (https://circuitdigest.com/forums/general/12v-dc-5v-dc-06a-lm317)

But I've been having heat issues with this component. In theory those LM317 are a nice option.

I think if thermally stable, they are ideal.