I don't know about electric cars, but for gas powered cars there are open source ECUs [0][1]. There are also tuners that directly modify the car's firmware to improve performance. Finally, you can connect a computer to the CAN bus [2], which allows you to capture and replay commands, as well as craft your own commands. This is how Comma's openpilot [3] works: it connects to the CAN bus and sends commands for all supported functionality.
What kind of features did you have in mind?
[0] https://en.wikipedia.org/wiki/Engine_control_unit
[1] https://rusefi.com/
[2] https://en.wikipedia.org/wiki/CAN_bus
[3] https://comma.ai/openpilot