Since InfinfiTime is an open source firmware, everybody can participate in the development or modify your custom firmware.
InfiniSim is an simulator for the InfiniTime/PineTime. That way you can test your code without risking to brick your actual watch.

Load custom Firmware into InfiniSim

If you build InfiniSim with the default configuration, you get a running simulator.
But It will not include your changes.

To do that you have to specify the -DInfiniTime_DIR=InfiniTime parameter in the build configuration.

[tux@server]$ cmake -S . -B build -DInfiniTime_DIR=/home/jannik/git/InfiniTime
[tux@server]$ cmake --build build -j4

That way InfiniSim will not only build it self but the included InfiniTime Firmware with all your changes as well.

If you now run InfiniSim you will see your changes.

[tux@server]$ ./build/infinisim