There are several types of circuit.
Simple circuits are those with few components, such as dividing down a signal with a small bunch of resistors
This can be turned into a more slightly more complicated circuit using inductors or capacitors to alter the ratio dependant upon the frequency.
Circuits of medium complexity include chips such as for timing, which need a few extra components to enable them to work correctly. This type may need to refer to the datasheet the work out what value of resistors and/or capacitors are needed for a particular delay or frequency.
Complicated circuits include those which need and operate with a microprocessor. This is because more modern versions are small, and need care to solder the pins onto a board.
There are also circuits which utilise an Arduinos. This allows the user to connect a PC to a circuit, and control some parameters from the PC/extract some data to the PC, or to have a the Arduino run some code that may alter on a more regular basis than a soldered in chip. Some versions allow the user to plug it onto breadboards, so be able to simulate an experiment or as some people would say 'real world experimentation'.