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If you can't tell by the title, I'm extremely new to PCB design software. I'm working on a senior project, and was hoping to design a custom PCB for our electronics. Currently, our circuits are spread out over a few breadboards and perfboards, but we'd like to combine them all onto one, custom board.

Questions: How can I create a schematic when several components we are using don't have models (TI SN754410, TI LM2576, and Adafruit nrf8001 bluetooth module). Since we've already built and tested the circuit, can't we just use blank DIP16 (TI SN754410) and TO220-5 (TI LM2576) models as placeholders? Is this what people refer to as "footprints"? We don't need to verify the design in software, we just want gerber files for a well laid out, compact board.
There are two parts to a component - the schematic symbol and the PCB footprint. While it is possible to make a board without a schematic (ie, place the appropriate footprints like a DIP16 or TO220-5 and draw traces between them), it is much better to start with a schematic. Most PCB CAD packages will have additional checking (called DRC - Design Rule Checks) such as verifying that you have connected everything on the board, and that nets don't have multiple drivers. These can only happen if it has the schematic as well. In addition, if you are using pours or ground planes, these can only connect to named nets from the schematic.

If you can't find a model for the component you are using, create your own. It's usually simple, and is an important step in layout. There should be tutorials online or in YouTube for whichever CAD package you are using.