The manufacturers give the 'User manual' for a gadget when we purchase it new. Sometimes the user manuals are available to be downloaded from the web site of the manufacturer.
User manuals help us in using the gadget for our daily or needed operations by that gadget. They also contain some tips on safety and even trouble shooting tips if something goes wrong. It will also have some technical specifications. For a general user these suffice. In case of need user can call the authorised service personnel of the manufacturer or a technician locally available for that.
If one needs to know the full working principles, the design and full technical details, purpose, functions and limitations of components, then one has to undergo formal education on the concerned branches of science and engineering. Even that may not be sufficient because each gadget and even each new model may need on the job training.
However for a person who has learned some basic principles in school and college in science, electronics and electricity, some more practical knowledge can be easily gained by referring to self tutorials and articles available in internet now a days.
There are local tutorial institutions giving short term package classes in servicing in mobile phones, computers and other gadgets.
A search like 'How gadgets work" or " How....(name of the gadget woks" can yield you results which can lead you to useful links and resources.
However after using and observing service technicians doing the repair and maintenance work on our gadgets, we can learn a lot.
There are easy DIY- Do It Yourself tutorials available in internet. They are available in text and audio-visual clips also.