Open source projects like Arduino and the Pyboard have schematics available. If you want to learn about about why those components are there read the datasheets for the major ICs, the board designer will usually be following the manufacturers recommendations.
Emulation is rarely ever done, it'd be a high effort project with a lot of nuance to specify external hardware and the hardware is cheap anyway. A rough flow for an embedded project would be to start with a development board from the MCU manufacturer to get a head start on firmware, move on to a custom dev board of your actual product but with more easily accessible pins and test points, and then iterate on the final formfactor board.
For a hobbyist start with an arduino and a pyboard connected to a breadboard. Once you're happy with the firmware and circuit design then start with the open source schematic and modify it to add your new components and lay it out on your custom board.
Emulation is rarely ever done, it'd be a high effort project with a lot of nuance to specify external hardware and the hardware is cheap anyway. A rough flow for an embedded project would be to start with a development board from the MCU manufacturer to get a head start on firmware, move on to a custom dev board of your actual product but with more easily accessible pins and test points, and then iterate on the final formfactor board.
For a hobbyist start with an arduino and a pyboard connected to a breadboard. Once you're happy with the firmware and circuit design then start with the open source schematic and modify it to add your new components and lay it out on your custom board.