This is a great guide by SparkFun on how to get started with OBD-II: https://learn.sparkfun.com/tutorials/getting-started-with-obd-ii
The most important aspect of OBD for making apps for your car is being able to read PIDs (Parameter IDs, or variables) from your car. Here’s a list of these PID signals List of Performance Information Data (PIDs) indicators - Carloop Community
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