Check out example codes for "vscode arduino [Warning] Output path is not specified. Unable to reuse previously compiled files. Upload could be slow. See README.". It will help you in understanding the concepts better.

Code Example 1

/*You need to specify the output path for the compilation of each sketch you compile. You can do that by pressing f1, typing in ".vscode/arduino.json" and adding the line specifying the address of the folder you want to use to keep the previous compilations. */ 	"output": "...../build"      //You need to do this for each sketch where you want to reuse the previous //compilations

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