This is what engineering is, building solutions by putting together individual subsystems and components based on accumulated knowledge. You build an electronic circuit the same way you build something like a house for example. You start with the foundation, then you build the floor, then you build and erect the walls and so on. You know you'll need windows, and doors and you know the basic rooms like a kitchen, bathroom, bedroom, etc and you decide how many, how large and where to put them. Then the plumbing and electrical, then the interior furnishings. Start with an overall vision, then break it down into subsections, then design each subsection. For example if I'm designing a widget I will start by defining what the widget is, what it's supposed to do, and any other requirements. Then I might start by selecting a microcontroller that I think will be adequate for the task, then I place the stuff I know it will need like power supply decoupling and a clock source, then I work on the power supply block, then whatever peripherals it needs to control and interface with the outside world. Many of these subsections will be similar to what I have used in other designs. Others will be based on concepts I have learned or parts I will fine by searching for components meant to perform a specific task, say controlling a stepper motor, making noises or displaying text on a grid of LEDs.