Please tell me, what should the ECU data flow and EGR be like?

Your tool is pretending to show you data that a gen 3 Prius does not supply. Where your tool thinks it is getting those numbers would be interesting to learn. It may have mistaken a Prius for some other kind of car.

 

You have said two things (that there are standard data, and that P0 codes are standardized) that are both correct, but not on point. The data you were showing in your photo in post 1 do not appear to correspond to any of the EGR data that a Prius will report. It seems likely the scan tool has a problem.

Here are the two live-data values the gen 3 Prius does in fact report pertaining to EGR:



Also, the reading from the MAP sensor (a reasonably standard PID) can be of use when diagnosing EGR.

Its usefulness is greatest when you have a scan tool that can issue active test commands, so that you can view the MAP reading response as you command changes to the EGR position. Without the ability to command changes, interpretation is difficult.

The most useful information you can get from the car about EGR is the result of the Mode 6 EGR test, where the car performs its own valve position changes, observes the MAP reading response, and summarizes the results for you as a test outcome.

 

The scanner shows EGR_PTC (%) of 0, which would correspond to the Target EGR Position 0%: Idling, which means the ECU thinks that the EGR valve is closed. This makes perfect sense.

The scanner shows EGR_ERR (%) of -100.0, does not correspond to the EGR Step Position shown in the Toyota Prius manual. So, the data shown in the scanner does not make any sense.

 

Then what does -100.0% mean? Why does it have a negative sign? Interpret to me what your scanner is saying in English.

All of the data that you can access through the OBD2 port is data that is located in the many ECUs in the car. ChapmanF looked at the Toyota service manual and copied the two parameters that are stored for the EGR valve in one of the ECUs. Your generic scanner is retrieving this data, and then it presents it on the scanner screen with a text description. The data for EGR_PTC seems to be accurate. Your scanner is not interpreting the data for the second parameter correctly. The second parameter can only have a valve from 0 to 120. It can not be -100.0%.

Wait until you have Techstream working and compare the data for the EGR valve between the scanner and Techstream.

There appears to be a few problems with the scanner that you are using.