reflash question

lets say in a hypothetical scenario, simply because i dont have a car yet, i want to access my cars stock ecu, how would i go about solving this 'issue'?

lets say in this same scenario i have myself the luxury of owning an au falcon from fb marketplace, and that its running smoothly (as this is a hypothetical scenario, it can be as perfect as i want and for simplicities sake, there was no damage when i got it, as if she was brand new).

also what are some things to avoid, some do's and dont's when trying to access and make changes to a stock ecu?

That varies per application. If it's an OBD2 vehicle, most often the OBD2 port is the point of access, using either a laptop and a cable, or a handheld device and a cable (potentially also cabled to a laptop).

Each vehicle application may have 0,1, to many companies that have figured out how to communicate with and reflash the ECU, plus possibly some open source options, so some searching online will hopefully reveal the products available for a particular vehicle.

HPA's reflash tuning course has extensive info on the reflash tuning process, so I suggest completing the course if you haven't already, as if will likely answer many questions you may have.

https://www.hpacademy.com/courses/practical-reflash-tuning/