How to build stuff with random components
One approach is to ignore the components and focus on all the IC's that exist on the board (i.e. encoder, decoder, adc, dac, eeprom, drivers, 555 timer)


Then look up their datasheets to see what kind of other application each individual can be used for. Many IC's datasheet do explain common applications that these components are used for, or google "[IC-Model] circuit applications" and see where that can take you


Other cases where these parts are sensor or motors/actuators, you just have to think about making other application that could use sensing or actuating abilities.


for example, you're taking parts from a lamp which uses an infrared sensor to detect when to turn the light on. So now you can take the infrared sensor, and reuse it as a trashcan opening sensor by having it detect nearby users. You can have a general purpose micro controller to test salvaged components