A balancing project help needed

Hi, I’ve been working on a balancing project. I’d like to make a platform that could be balanced by an arduino (in my case an ESP32) car. The main idea was that I’ll use 2 ESP board, one for measure the angle of the platform that can be rotated arount one axis, and the second board which would be the car, and they would communicate via WiFi (ESP-NOW). I will attach a link below about it.

Already done:
I can read the MPU6050 and convert the values into angle and send the angle value to the another ESP.
I can control the encoded DC motors of the car with L298N, I can tell the car for example go froward or beckward 60 steps.

What is my problem:
I’ve some problem with the regulation, actually I’ve no clue how could I do the regulation. I would like to use PID, because it’s simple, but I haven’t done PID control before.
The main question would be, for what should I regulate if I know the error angle? Steps, speed or what of the motors?

And finally here is the link, watch from 1:05 to 1:16. I’ll use only one robot, and my will only goes to and fro straight.

Thank you in advance!